…ence

Second slice of the Slack app work. Stands up the OAuth handshake,
workspace persistence with encrypted bot tokens, and the four routes
the dashboard needs to render the connect/disconnect/status UI.

Crypto refactor (no behaviour change):
- Extract encryptString / decryptString from llm-key-crypto.ts as the
  generic primitive (HKDF-SHA256 sub-key derivation + AES-256-GCM).
- LLM key wrappers (encryptLlmKey / decryptLlmKey) now build their salt
  from the existing KeyScope and call into the generic primitive.
- New Slack wrappers (encryptSlackBotToken / decryptSlackBotToken) use
  salt='slack:<slack_team_id>' so each workspace's token has a distinct
  derived sub-key.

slack-workspace-service.ts:
- upsertSlackWorkspace (create or re-install), getActiveWorkspaceForTeam
  / forUser, getWorkspaceBySlackTeamId, getWorkspaceWithToken (decrypts
  the bot token; only used when actually calling Slack), softDelete-
  Workspace. All audit-logged: slack.installed, slack.reinstalled,
  slack.uninstalled.

slack-oauth.ts:
- signOauthState / verifyOauthState — opaque, AEAD-encrypted state with
  a 10-min TTL, carries the originating reflect_user_id. Reuses the
  master encryption key (no extra env var).
- loadSlackOauthConfig + getActiveSlackConfig — reads REFLECT_DEV_SLACK_*
  or REFLECT_PROD_SLACK_* depending on RM_SLACK_ENV.
- buildInstallUrl — composes the slack.com/oauth/v2/authorize URL with
  the bot scope set the v1 manifest declares.
- exchangeOauthCode — calls slack.com/api/oauth.v2.access with the code,
  returns workspace metadata + bot token on success.
- verifySlackSignature — HMAC-SHA256 with timing-safe compare and a
  5-min timestamp tolerance, ready for /slack/events in phase 3.
- revokeSlackToken — best-effort auth.revoke during uninstall.

HTTP routes (4 new, all admin-gated except the public callback):
- POST   /slack/install-url       -> { url, state, redirect_uri }
- GET    /slack/oauth/callback    -> 302 to dashboard ?installed=1
                                     (signed state replaces auth)
- GET    /slack/status            -> { configured, workspace | null }
- DELETE /slack/uninstall         -> 200 + audit, best-effort revoke

The /slack/oauth/callback bypass in the global Bearer-auth hook is
explicit; the signed state IS the auth.

Tests: +20 (335 total, all green).
- State HMAC roundtrip / tampering / expiry / bogus input.
- verifySlackSignature: valid / wrong sig / stale / future / non-numeric.
- POST /slack/install-url: 403 for non-admin, returns slack.com URL with
  the state echoed and parseable.
- GET /slack/status: 403 for non-admin, null when nothing installed,
  populated after a direct upsert (cross-process master key shared via
  .test-server.json so this works).
- GET /slack/oauth/callback error paths: missing code, bad state, slack
  error param all redirect to dashboard with error= query param.
- DELETE /slack/uninstall: 403 / 404 / 200 + audit event verified.

Refs: parent memory d959bc61 (Eng Plan: Slack App v1) + 3a2f27a3
(Phase 1 shipped).

Made-with: Cursor