feat(connectors): external-source connector layer + GitHub Issues (#57)

Make Vestige a durable, local, semantically-searchable retrieval layer over an
external system of record (GitHub Issues first), citing back to the canonical
record. Unlike a live ticket-system MCP proxy, Vestige keeps a durable embedded
index: searchable offline, joinable with the rest of memory, temporally
versioned, and re-syncable idempotently with no duplication.

Phases 1-2 of #57 plus a GitHub reference connector and source-aware search:

- Source envelope on KnowledgeNode/IngestInput (source_system, source_id,
  source_url, source_updated_at, content_hash, synced_at, source_project,
  source_type, source_author). Migration V17: nullable columns (additive),
  partial UNIQUE index on (source_system, source_id), connector_cursors table.
- Idempotent sync primitives in vestige-core: upsert_by_source (content-hash
  change detection), connector cursor checkpoints, reconcile_source_tombstones
  (invalidate-don't-delete via bitemporal valid_until).
- Connector contract + run_sync driver + GitHub Issues connector behind the
  optional `connectors` feature (on by default in vestige-mcp, off in the core
  library default so non-connector consumers link no HTTP client).
- source_sync MCP tool ({"repo": "owner/name"}); token from GITHUB_TOKEN env
  only. Search results gain a sourceRecord citation for connector memories.

Adversarial review fixes: GitHub `since` Z-form (the `+00:00` offset corrupted
the cursor server-side), un-tombstone clears superseded_by too, cursor never
advances past a failing record, Link next-url host-pinned (token-leak guard),
records_seen counts new records only.

Verified: cargo check/test/clippy -D warnings green across the workspace
(default and connectors features); 483 core tests pass. Version bump to 2.1.27
and tag deferred to release.

Refs #57

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/e2e/src/harness/db_manager.rs

@@ -31,6 +31,7 @@ fn make_ingest_input(
         source,
         valid_from,
         valid_until,
+        source_envelope: None,
     }
 }
 

tests/e2e/src/mocks/fixtures.rs

@@ -29,6 +29,7 @@ fn make_ingest_input(
         source,
         valid_from,
         valid_until,
+        source_envelope: None,
     }
 }