Tasks: Server-side input queue for auto-resume

从 coderd/aitasks.go 和针对 task_pending_inputs 的新 migration 开始，然后查看所引用的生命周期 RFC 以及现有的 job-system 集成。跟踪 /send 如何处理已暂停的 task，以及 workspace 就绪状态和投递是如何表示的。完成的标准是：排队的消息返回带有 ID 的 202，在 workspace 自动启动后完成投递，并按规定记录失败状态。

Repo: coder/coder

Scope: Allow /send to accept messages for paused tasks, queue them, and deliver asynchronously.

Note: Consider ACP (Agent Communication Protocol) impact before implementation. ACP standardization may influence message queue design and delivery mechanism.

Changes:

• Create task_pending_inputs table:
  • id UUID PRIMARY KEY
  • task_id UUID REFERENCES tasks(id)
  • content TEXT NOT NULL
  • status TEXT NOT NULL DEFAULT 'pending'
  • created_at TIMESTAMPTZ NOT NULL
  • delivered_at TIMESTAMPTZ
  • error TEXT
• Modify /send to accept messages for paused tasks
• Return 202 Accepted with message ID for queued messages
• Background worker: trigger workspace start, poll for ready, deliver message, update status
• Handle failures (build fails, delivery fails) with appropriate status updates

Benefits beyond auto-resume (per RFC):

• Solves Terraform env var length limits for long prompts
• Unifies initial prompt + follow-up delivery path
• Enables prompt modification until task starts
• Enables prebuilds for tasks

Files:

• New migration for task_pending_inputs table
• coderd/aitasks.go (modified send handler)
• New background worker or integration with existing job system

Acceptance criteria:

• /send to paused task returns 202 and queues message
• Workspace automatically starts
• Message delivered when workspace ready
• Failure states handled gracefully with status tracking
• Direct API consumers no longer need orchestration logic

Dependencies:

• GA phase complete
• ACP (Agent Communication Protocol) evaluation (does not block, but may influence design)

References:

• PRD: Start/Pause/Resume the Task Workspace
• RFC: Tasks: Start/Pause/Resume Lifecycle: Auto-Resume Option B