P4: Slack inbound @mention — broker-verified signatures drive real runs

- Broker op VerifySlackSignature: v0 HMAC-SHA256 computed INSIDE the broker
  (constant-time compare); the signing secret never crosses the socket.
  Slack secrets are one JSON credential {bot_token, signing_secret}; the
  broker extracts the right field per operation
- Public POST /api/slack/events: signature verified against connected slack
  connections via the broker; forged signatures 401; url_verification
  handshake echoed only when signed; app_mention starts a real run in the
  agent's dedicated '💬 Slack' session — and the agent's reply is itself a
  gated outbound post
- SlackApp Connection tab captures bot token + signing secret
- Integration test: forged 401, signed challenge, signed mention -> run ->
  slack.post pending in the approval queue
- E2E: full loop — connect, gated outbound (sink empty -> exactly one post),
  then a node-crypto-signed mention -> approval card -> approve -> 'On it!'
  lands in the sink

134 Rust + 63 frontend tests + 21 Playwright journeys.

Co-Authored-By: Claude Fable 5 <[email protected]>

crates/tc-secrets/Cargo.toml

@@ -9,6 +9,8 @@ publish.workspace = true
 [dependencies]
 chacha20poly1305 = "0.10"
 hex = "0.4"
+hmac = "0.12"
+sha2 = "0.10"
 reqwest = { version = "0.12", default-features = false, features = [
     "json",
     "rustls-tls",