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.