Fail fast on Slack auth hangs

[manu354]

Adds bounded timeouts for agent-facing Slack commands so CLI calls do not hang silently when keychain, Slack API, or rate-limit retry paths stall.

Changes:

• Add Slack API request timeout env override via AGENT_SLACK_API_TIMEOUT_MS.
• Fail fast on browser-auth 429 retry windows by default, with opt-in wait via AGENT_SLACK_RATE_LIMIT_MAX_WAIT_MS.
• Bound macOS keychain / safe-storage subprocess calls via AGENT_SLACK_KEYCHAIN_TIMEOUT_MS.
• Add a non-interactive command watchdog via AGENT_SLACK_COMMAND_TIMEOUT_MS.
• Add regression coverage for browser API timeout and 429 fail-fast behavior.

Validation:

• bun run typecheck
• bun test (241 pass)
• bun run lint (0 errors; existing warnings only)
• bun run format:check
• bun run build:npm
• node --check dist/index.js
• Built CLI smoke against brain-grf4627 auth test/message list now exits quickly with invalid_auth instead of hanging.

[nwparker]

✅ Taken over, verified, and merged into main via #121 (commit 80f0e8d).

Branch takeover/pr-110. There was a conflict in src/slack/client.ts (main had refactored browserApiMultipart to an input object and added a 429-retry loop). Resolved by combining your timeout wrap with that refactor.

Heads-up (deliberate, disclosed): I extended your 429 fail-fast to the multipart upload path too, so both transports honor the "no hangs" promise (previously multipart could still wait up to 30s). The existing test/client.test.ts "retries HTTP 429" test now opts into waiting via AGENT_SLACK_RATE_LIMIT_MAX_WAIT_MS=30000 — retry logic unchanged, just gated behind the opt-in your PR introduces.

Verified: typecheck clean · 345 tests / 0 fail · bun test test/search-files.test.ts → 4 pass (timeout + 429 fail-fast) · lint 0 errors · build OK. Your commit 342b76d is preserved. Thanks @manu354! Closing as merged.