Add OpenCode server cooperative execution

[drafael]

Summary

• add explicit opencode-server implementation mode while preserving direct opencode execution and review
• own one authenticated, auto-update-disabled loopback server and dedicated session per attempt
• bind exact server-instance/session/message identity and require uninterrupted SSE plus fresh REST terminal reconciliation
• accept cancellation only after exact MessageAbortedError; fail closed on continuity, identity, protocol, or interactive-authority loss
• share awaited direct-child cleanup with Codex app-server and document the whole-harness-loss boundary

Validation

• npm test: 234 tests, 232 passed, 2 platform skips
• typecheck, lint, format check, Markdown links, and git diff --check
• deterministic dist/: 100 files across repeated builds
• package dry-run: 103 files; no project-owned .exe, .node, or dist/native/
• live OpenCode 1.18.28 completion and cancellation:
  • exact opencode-server@1.18.28 subject
  • requested result.txt contained validation\n
  • cancellation reconciled to exact aborted assistant and exit 130
  • ~/.config/opencode digest unchanged
  • no OpenCode server remained after awaited cleanup

Boundaries

• no SSE replay, reconnect, or restart reattachment
• whole-harness loss is EXECUTION_STATE_UNKNOWN and may leave provider processes for operator recovery
• cooperative terminality does not claim descendant quiescence, sandboxing, external-effect rollback, or absence of persistent OpenCode data