api/sdk: optional Idempotency-Key support for messaging writes

[willwashburn]

Summary

• add server-side idempotency handling via Idempotency-Key for write endpoints:
  • POST /v1/channels/:name/messages
  • POST /v1/messages/:id/replies
  • POST /v1/dm
  • POST /v1/dm/:conversation_id/messages
• introduce shared middleware helper (runIdempotent) backed by Redis with:
  • replayed response return + Idempotency-Replayed: true
  • conflict on same key with different payload (409 idempotency_key_reused)
  • conflict while in-flight (409 idempotency_in_progress)
  • graceful degradation when Redis methods are unavailable
• extend SDK HTTP client to accept per-request headers and expose optional idempotencyKey on agent messaging APIs (send, reply, dm, dms.sendMessage)
• add server middleware tests and SDK header-propagation tests
• document idempotency usage in README authentication section
• include existing .gitignore update (.npm-cache) per request

Validation

• npm --prefix /Users/will/Projects/relaycast/packages/server test -- src/middleware/__tests__/idempotency.test.ts src/routes/__tests__/message.test.ts src/routes/__tests__/dm.test.ts src/routes/__tests__/thread.test.ts src/routes/__tests__/groupDm.test.ts
  • passed (33/33)
• npm --prefix /Users/will/Projects/relaycast/packages/sdk test -- src/__tests__/agent-messaging.test.ts
  • passed (23/23)

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[devin-ai-integration]

Devin Review found 2 potential issues.

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[devin-ai-integration]

Devin Review found 1 new potential issue.

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