feat(opencode): add hook-only installation mode

[zrh805]

Description

Adds an opt-in hook-only mode for the OpenCode plugin. The default remains unchanged: the plugin registers its bundled stdio MCP proxy.

Set mcp.enabled to false in openviking-config.json to retain repository context, automatic recall, message capture, and lifecycle commits without adding or overwriting OpenCode's mcp.openviking entry.

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

Fixes #3690

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Add mcp.enabled, defaulting to true, to the OpenCode plugin configuration.
• Skip bundled MCP registration without mutating existing OpenCode MCP configuration when hook-only mode is enabled.
• Document the option in English and Chinese installation guides, with configuration, injection, and plugin-entrypoint regression tests.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

Validated with npm test (23 passing tests) and npm run check in examples/opencode-plugin.