Add OpenCode agent support and allow enable without hook installation

[MementoMori123]

Summary

Adds OpenCode as a supported agent so Entire can be used in OpenCode projects. OpenCode is detected via .opencode/ or opencode.json. Because OpenCode is expected to integrate via its own plugin (calling entire hooks ...), this agent does not install or manage hooks; the setup flow is updated so entire enable can succeed for such agents.

Changes

• New agent: OpenCode (cmd/entire/cli/agent/opencode/)

  • Implements the Agent interface: detection, session ID transform, session dir resolution, protected dirs (.opencode), and resume command.
  • SupportsHooks() returns false; hook input parsing is unimplemented (returns a clear error if used).
  • Session read/write is implemented as a passthrough of OpenCode’s on-disk format so future hook/plugin integration can be added without changing this contract.

• Setup behavior

  • Agents that do not support hooks (e.g. OpenCode) can complete entire enable without installing hooks. Entire’s git hooks and strategy setup still run; only agent-specific hook installation is skipped.
  • Message when no hooks are installed is updated to: “Hooks for … already installed or managed externally” so it’s accurate for both “already installed” and “managed by the agent/plugin elsewhere.”

• Registration

  • OpenCode is registered in the agent registry and wired in config.go and hooks_cmd.go so it is discoverable and usable when OpenCode (or its plugin) invokes the CLI.

Testing

• mise run test (unit tests including new registry test for opencode).
• Manual: entire enable in a repo with .opencode/ or opencode.json completes successfully and does not require hook support.