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

hooks/scripts/check-config.sh

@@ -97,7 +97,16 @@ if [[ -n "$HAS_BSKY" ]]; then
   SOURCE_COUNT=$((SOURCE_COUNT + 1))
 fi
 if [[ -n "$HAS_SCRAPECREATORS" ]]; then
-  SOURCE_COUNT=$((SOURCE_COUNT + 3))  # Reddit comments + TikTok + Instagram
+  # Start with Reddit comments + TikTok + Instagram, subtract any in EXCLUDE_SOURCES
+  SC_ADD=3
+  EXCLUDED="${ENV_EXCLUDE_SOURCES:-${EXCLUDE_SOURCES:-}}"
+  if [[ ",$EXCLUDED," == *",tiktok,"* ]]; then
+    SC_ADD=$((SC_ADD - 1))
+  fi
+  if [[ ",$EXCLUDED," == *",instagram,"* ]]; then
+    SC_ADD=$((SC_ADD - 1))
+  fi
+  SOURCE_COUNT=$((SOURCE_COUNT + SC_ADD))
 fi
 
 if [[ -n "$HAS_SCRAPECREATORS" ]]; then