feat: implement RedditCommentStrategy and register in platform registry

Adds RedditCommentStrategy to app/services/platforms/reddit_comment.py,
resolving thread_id via thread_url_override or _find_sticky title search,
falling back to reconstructed URL when client.comment() returns empty string.
Registers the strategy under "reddit_comment" in the platform _REGISTRY.
7 new tests confirm all execution paths: url override, title pattern lookup,
not-found error, missing-extra error, empty-URL reconstruction, dupe guard,
and registry presence. Full suite: 34/34 passing.

app/services/platforms/__init__.py

@@ -2,12 +2,13 @@ from __future__ import annotations
 
 from app.services.platforms.base import PostingStrategy, PostResult
 from app.services.platforms.reddit_post import RedditPostStrategy
+from app.services.platforms.reddit_comment import RedditCommentStrategy
 
 _REGISTRY: dict[str, PostingStrategy] = {
     s.campaign_type: s()
     for s in [
         RedditPostStrategy,
-        # RedditCommentStrategy — added in Plan B
+        RedditCommentStrategy,
         # BlogPostStrategy — added in Plan C
     ]
 }

app/services/platforms/reddit_comment.py

@@ -7,6 +7,8 @@ from datetime import date, timedelta
 import httpx
 
 from app.services.platforms.base import PostingStrategy, PostResult
+from app.services.reddit.client import RedditClient
+from app.core.config import get_settings
 
 logger = logging.getLogger(__name__)
 
@@ -85,3 +87,51 @@ def _find_sticky(
         if pattern_lower in title.lower():
             return post.get("id")
     return None
+
+
+class RedditCommentStrategy(PostingStrategy):
+    campaign_type = "reddit_comment"
+
+    def supports_dupe_guard(self) -> bool:
+        return False  # comment threads may appear multiple times
+
+    def execute(
+        self,
+        *,
+        target: str,
+        title: str,
+        body: str,
+        flair: str | None = None,
+        extra: dict | None = None,
+    ) -> PostResult:
+        extra = extra or {}
+        thread_url_override = extra.get("thread_url_override")
+        thread_title_pattern = extra.get("thread_title_pattern")
+        session_file = get_settings().reddit_session_file
+
+        # Resolve thread_id
+        if thread_url_override:
+            thread_id = _extract_thread_id_from_url(thread_url_override)
+        elif thread_title_pattern:
+            thread_id = _find_sticky(
+                sub=target,
+                title_pattern=thread_title_pattern,
+                session_file=session_file,
+            )
+            if thread_id is None:
+                raise ValueError(
+                    f"No thread matching {thread_title_pattern!r} found in r/{target}"
+                )
+        else:
+            raise ValueError(
+                "RedditCommentStrategy requires thread_url_override or thread_title_pattern in extra"
+            )
+
+        client = RedditClient(session_file=session_file)
+        comment_url = client.comment(thread_id=thread_id, body=body)
+
+        # Reddit comment() may return empty URL; reconstruct from thread_id
+        if not comment_url:
+            comment_url = f"https://www.reddit.com/r/{target}/comments/{thread_id}/"
+
+        return PostResult(url=comment_url, metadata={"thread_id": thread_id})

tests/services/platforms/test_reddit_comment_strategy.py

@@ -0,0 +1,90 @@
+from unittest.mock import patch, MagicMock
+import pytest
+from app.services.platforms.reddit_comment import RedditCommentStrategy
+from app.services.platforms.base import PostResult
+
+
+@pytest.fixture
+def strategy():
+    return RedditCommentStrategy()
+
+
+def test_execute_with_url_override(strategy, monkeypatch):
+    """Uses thread_url_override to get thread_id, calls client.comment()"""
+    mock_client = MagicMock()
+    mock_client.comment.return_value = "https://www.reddit.com/r/flipping/comments/abc123/_/xyz789/"
+    with patch("app.services.platforms.reddit_comment.RedditClient", return_value=mock_client):
+        with patch("app.services.platforms.reddit_comment.get_settings") as mock_settings:
+            mock_settings.return_value.reddit_session_file = "/fake/session.json"
+            result = strategy.execute(
+                target="flipping",
+                title="ignored",
+                body="Hello thread!",
+                extra={"thread_url_override": "https://www.reddit.com/r/flipping/comments/abc123/weekly/"},
+            )
+    assert result.url == "https://www.reddit.com/r/flipping/comments/abc123/_/xyz789/"
+    assert result.metadata["thread_id"] == "abc123"
+    mock_client.comment.assert_called_once_with(thread_id="abc123", body="Hello thread!")
+
+
+def test_execute_with_title_pattern_found(strategy):
+    """Uses _find_sticky to locate thread, posts comment"""
+    with patch("app.services.platforms.reddit_comment._find_sticky", return_value="def456"):
+        with patch("app.services.platforms.reddit_comment.RedditClient") as MockClient:
+            with patch("app.services.platforms.reddit_comment.get_settings") as mock_settings:
+                mock_settings.return_value.reddit_session_file = "/fake/session.json"
+                MockClient.return_value.comment.return_value = ""
+                result = strategy.execute(
+                    target="cscareerquestions",
+                    title="ignored",
+                    body="Job search tool",
+                    extra={"thread_title_pattern": "Monthly Resume"},
+                )
+    assert "def456" in result.url
+    assert result.metadata["thread_id"] == "def456"
+
+
+def test_execute_thread_not_found(strategy):
+    """Raises ValueError when _find_sticky returns None"""
+    with patch("app.services.platforms.reddit_comment._find_sticky", return_value=None):
+        with patch("app.services.platforms.reddit_comment.get_settings") as mock_settings:
+            mock_settings.return_value.reddit_session_file = "/fake/session.json"
+            with pytest.raises(ValueError, match="No thread matching"):
+                strategy.execute(
+                    target="cscareerquestions",
+                    title="ignored",
+                    body="body",
+                    extra={"thread_title_pattern": "Monthly Resume"},
+                )
+
+
+def test_execute_no_extra_raises(strategy):
+    """Raises ValueError when neither thread_url_override nor thread_title_pattern provided"""
+    with patch("app.services.platforms.reddit_comment.get_settings") as mock_settings:
+        mock_settings.return_value.reddit_session_file = "/fake/session.json"
+        with pytest.raises(ValueError, match="requires thread_url_override or thread_title_pattern"):
+            strategy.execute(target="flipping", title="t", body="b", extra={})
+
+
+def test_reconstructed_url_on_empty_comment_url(strategy):
+    """When client.comment() returns empty string, reconstructs URL from thread_id"""
+    with patch("app.services.platforms.reddit_comment.RedditClient") as MockClient:
+        with patch("app.services.platforms.reddit_comment.get_settings") as mock_settings:
+            mock_settings.return_value.reddit_session_file = "/fake/session.json"
+            MockClient.return_value.comment.return_value = ""
+            result = strategy.execute(
+                target="flipping",
+                title="t",
+                body="b",
+                extra={"thread_url_override": "https://www.reddit.com/r/flipping/comments/abc123/weekly/"},
+            )
+    assert result.url == "https://www.reddit.com/r/flipping/comments/abc123/"
+
+
+def test_supports_dupe_guard_false(strategy):
+    assert strategy.supports_dupe_guard() is False
+
+
+def test_registry_contains_reddit_comment():
+    from app.services.platforms import _REGISTRY
+    assert "reddit_comment" in _REGISTRY