feat: honor EXCLUDE_SOURCES env var in source count + pipeline filter

Adds a per-run denylist via the existing-but-unused EXCLUDE_SOURCES
config key. Two coupled changes:

1. pipeline.available_sources() filters out any source listed in
   config["EXCLUDE_SOURCES"] (comma-separated, case-insensitive,
   whitespace-tolerant) before returning.
2. hooks/scripts/check-config.sh "Ready — N sources active" banner
   subtracts excluded sources from the ScrapeCreators +3 (Reddit
   comments + TikTok + Instagram) so the count matches what the
   pipeline actually runs.

Use case: skip TikTok/Instagram on runs where you only want
text-substantive sources, without unsetting SCRAPECREATORS_API_KEY
(which would also kill Reddit comments). The existing INCLUDE_SOURCES
allowlist covers Perplexity opt-in but doesn't cover this denylist case
— tiktok and instagram are added unconditionally when
SCRAPECREATORS_API_KEY is set, with no opt-out short of removing the key.

Tests (tests/test_pipeline_v3.py::TestExcludeSources):
- excludes tiktok+instagram when listed
- no exclusion when env unset or empty string
- case-insensitive + whitespace-tolerant parsing
- works for any source (e.g. EXCLUDE_SOURCES=hackernews), not just SC-backed

skills/last30days/scripts/lib/pipeline.py

@@ -128,6 +128,9 @@ def available_sources(config: dict[str, Any], requested_sources: list[str] | Non
         available.append("pinterest")
     if env.is_xquik_available(config):
         available.append("xquik")
+    exclude = {s.strip().lower() for s in (config.get("EXCLUDE_SOURCES") or "").split(",") if s.strip()}
+    if exclude:
+        available = [s for s in available if s not in exclude]
     return available