[chore] Remove `_make_legacy_git_wiki_id` migration helper

[arozumenko]

Context

Follow-up to #117 / #195. PR #195 changed _make_wiki_id from hashing f"{repo_url}:{branch}" to hashing {source_type, scope} canonically. To avoid orphaning pre-#195 git wikis, _make_legacy_git_wiki_id was added as a fallback: when the new-style ID doesn't exist in the DB but the old-style does, the service uses the legacy ID.

This is a one-cycle migration helper. Once all installations have run at least once on the new code (so legacy IDs are no longer needed because every wiki has been re-keyed), the helper should be removed.

Acceptance

• Delete _make_legacy_git_wiki_id from backend/app/services/wiki_service.py
• Delete the legacy-ID fallback lookup in WikiService.generate() (around lines 280-287 in feat(integration): #189 — Generate Wiki dialog + multi-source ingestion wiring #195's diff)
• Delete the unit test that exercises legacy-ID compat
• Add a release-notes entry noting the legacy-ID grace period is over

Timing

Wait until at least one stable release post-#195 has been in the wild for ~30 days, or after the next milestone bump — whichever the maintainer prefers. Don't merge this PR before then.

Out of scope

• Data migration script — not needed. Wikis re-keyed on the next generation cycle; orphaned legacy IDs in the DB become unreachable but don't cause errors.

Parent

Follow-up to #117 / #195.