Fix duplicate write handling in opencode mode

[Nomadcxx]

Summary

• Stop registering the native write hook in opencode tool-loop mode, so opencode owns native write execution.
• Keep oc_write available, and keep native write registration for proxy-exec and off modes.
• Add one shared write-tool contract that points targeted existing-file changes to edit.

Notes

Issue #83 started around missing oc_* hooks. The current failure traced to duplicate native write handling after aliases existed: opencode completed write, then the plugin guard could return a partial-overwrite error and keep the model looping.

Manual Composer checks against the dev plugin did not reproduce the duplicate-write guard loop. Separate prompts stalled after read or produced malformed edit args; those need a separate patch if we can isolate a plugin-side cause.

Fixes #83

Testing

• bun test tests/integration/opencode-loop.integration.test.ts tests/tools/defaults.test.ts tests/unit/plugin-tool-resolution.test.ts tests/unit/plugin-tools-hook.test.ts
• bun run test:ci:unit
• bun run build