fix(sources): align SC source gating between code and docs

Two related drifts surfaced while reviewing PR #399 (EXCLUDE_SOURCES) —
docs claimed several SC-backed sources required INCLUDE_SOURCES opt-in
that the code didn't actually enforce, and threads was inconsistently
gated relative to its same-key siblings.

This commit picks the "code as source of truth + EXCLUDE_SOURCES as
suppression knob" model and aligns docs to match. It also promotes
threads to the same auto-on tier as tiktok and instagram, since all
three share the SC key and per-call cost shape — there was no real
product reason for threads being opt-in while the other two weren't.

The resulting source-gating model is three-tier and intentional:

  • **Auto-on if backing infra present** (suppress via EXCLUDE_SOURCES):
    reddit, HN, polymarket, X, youtube, github, bluesky, truthsocial,
    grounding, **tiktok, instagram, threads**

  • **INCLUDE_SOURCES persistent opt-in** (cost/billing reasons):
    perplexity (different paid API — OpenRouter),
    tiktok_comments / youtube_comments (N× extra SC calls per video)

  • **--search per-query opt-in** (relevance reasons):
    pinterest (visual pins, narrow utility),
    xiaohongshu (Chinese-market specific)

Changes:

- env.py: `is_threads_available()` drops the INCLUDE_SOURCES check,
  now mirrors tiktok/instagram (SC key → True). Docstring updated.
- tests/test_env_v3.py: new `ThreadsAvailabilityTests` class locks in
  the new contract and includes a regression guard ("INCLUDE_SOURCES
  should not be needed").
- SKILL.md: lines 333-338 rewritten so the model's "Build
  ACTIVE_SOURCES_LIST" checklist reflects what the engine actually
  runs. Drops false INCLUDE_SOURCES requirement for
  tiktok/instagram/threads; corrects pinterest to mention --search;
  adds missing INCLUDE_SOURCES=perplexity requirement.
- README: same alignment for the user-facing "Everything else in v3"
  section.

Note on EXCLUDE_SOURCES references in the new docs: the suppression
flag is wired up in PR #399. SKILL.md and README mention EXCLUDE_SOURCES
as the opt-out path; that prose is forward-looking until #399 lands.
The behavior changes in this PR (threads auto-on) are self-contained
and don't require #399 to function — but for users who want to suppress
the newly-auto-on threads source, #399 needs to land first.

tests/test_env_v3.py

@@ -42,5 +42,25 @@ class EnvV3Tests(unittest.TestCase):
                 self.assertIsNone(bird_x.is_bird_authenticated())
 
 
+class ThreadsAvailabilityTests(unittest.TestCase):
+    """Threads is in the SC default-on family: same key, same per-call cost
+    shape as TikTok / Instagram, so the same default-on rule applies.
+    Suppression goes through EXCLUDE_SOURCES, not gated opt-in."""
+
+    def test_threads_available_with_sc_key_only(self):
+        self.assertTrue(env.is_threads_available({"SCRAPECREATORS_API_KEY": "k"}))
+
+    def test_threads_unavailable_without_sc_key(self):
+        self.assertFalse(env.is_threads_available({}))
+        self.assertFalse(env.is_threads_available({"INCLUDE_SOURCES": "threads"}))
+
+    def test_threads_does_not_require_include_sources(self):
+        """Regression guard: INCLUDE_SOURCES should not be needed."""
+        self.assertTrue(env.is_threads_available({
+            "SCRAPECREATORS_API_KEY": "k",
+            "INCLUDE_SOURCES": "",
+        }))
+
+
 if __name__ == "__main__":
     unittest.main()