feat: HTTP status endpoint (Bun.serve) replaces file-based status

[four-bytes-robby]

Closes #107

Pattern

Same as limits-sidebar: TUI polls a local HTTP endpoint for status data.

Architecture

• Bun.serve({ port: 0 }) — dynamic port, no collision risk
• Port written to ~/.cache/opencode/brain/status-port-<hash>.json (session-scoped)
• TUI reads port file once, then polls http://127.0.0.1:<port>/status every 200ms
• Status JSON served from memory — no file I/O for status updates
• stopStatusServer() called on plugin dispose

Files

• src/status.ts — startStatusServer/stopStatusServer + Bun.serve
• src/tui.tsx — HTTP fetch polling
• src/four-opencode-brain.ts — dispose hook
• src/shared.ts — emptied (getBrainStatusFile removed)

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Summary by cubic

Replaces file-based brain status with a local HTTP endpoint via Bun.serve on a dynamic port. Implements #107 by having the TUI resolve the port once and poll /status for in-memory JSON; removes status file I/O and shuts down the server on dispose.

• Refactors
  • Serve status at http://127.0.0.1:<port>/status via Bun.serve({ port: 0 }) (OS-assigned free port).
  • Write discovery file at ~/.cache/opencode/brain/status-port-<hash>.json (session-scoped).
  • TUI reads the port file once and polls the endpoint every 200ms.
  • Removed status file writes and getBrainStatusFile; added stopStatusServer() and a dispose hook.

^{Written for commit 837ecd0. Summary will update on new commits.}