feat: persist handle/email → Slack user-id resolution cache

[arnaud-gp]

Problem

resolveUserId() always hit Slack: users.lookupByEmail for emails, then a full users.list scan for handles. Browser xoxc tokens cannot call users.lookupByEmail (not_allowed_token_type), so every email lookup burned a failed request and then paginated the directory. In a workspace of this size that scan is multiple users.list pages, on every user get, user dm-open, channel invite, and search --user.

The existing users-cache-*.json file only stored id → profile for --resolve-users output annotation. It was never consulted in the reverse direction.

What Changed

• The per-workspace user cache now also stores handle/email → user-id aliases (24h TTL), written through from users.list pages and users.info results.
• resolveUserId() reads that index before calling Slack.
• Browser auth skips users.lookupByEmail.
• agent-slack user cache warm and user list --refresh prime the whole directory once.
• user get --refresh-users bypasses TTL so renames/deactivations are recoverable.
• README documents cache path, TTL, warm, and invalidation.

Verification

• bun test — 406 pass, 0 fail. New coverage: cache hit, miss with write-through, TTL expiry, browser-auth email short-circuit, user cache warm, and --refresh-users bypass.
• bun run typecheck — clean.

Risks

• Cache writes remain best-effort. A failed write still falls back to users.list.
• Stale handles until TTL or --refresh-users. Same 24h window as the existing profile cache.

Made with Cursor