* Replace hardcoded GitHub-email map with Store-backed user mapping
Move the static GITHUB_USER_EMAIL_MAP to a Store-backed bidirectional
mapping (GitHub login <-> work email <-> optional Slack ID) with an
in-process cache, self-service onboarding, and admin management.
- agent/dashboard/user_mappings.py: Store CRUD + login/email/slack-id
indexes, sync cache readers for hot paths, async fallthrough, and a
bulk_import that preserves existing richer records.
- Migrate all read sites (auth.py, agent_overrides.py, authorship.py,
github_comments.py, webapp.py x2) off the dict.
- Unmapped Slack tags now run on the GitHub App installation token
(use_installation_token_fallback) and get an ephemeral "link your
GitHub account" prompt carrying the Slack id + email via a signed
account-link token threaded through the OAuth state.
- OAuth callback completes a self-service (org-gated) mapping from that
token, falling back to the verified GitHub email.
- Admin CRUD endpoints + one-time legacy import; dashboard UI section.
- Legacy dict retained only as the import payload (no longer read).
Tests: mapping store, account-link round-trip + completion, mapped vs
unmapped Slack flows; existing trust-gate tests updated to prime cache.
* Address review: cold-cache email resolution + stale alias de-indexing
- agent_overrides: add resolve_login_from_email_async that falls through to
the Store on a cold cache; use it at the async repo-resolution call sites
(Slack repo config, Linear comment, owner-metadata) so a mapped user still
resolves to their GitHub login + dashboard default_repo on a fresh worker.
- user_mappings.upsert_mapping: de-index the existing login before re-indexing
so a changed email/Slack id no longer leaves stale aliases resolving to the
login in-process.
- Tests for both fixes; update Slack repo-config test to patch the async resolver.
