Use managed exact plugin versions for auto updates

[biggameking]

Summary

• install the plugin as a managed exact version instead of writing dynamic @latest entries
• persist channel metadata so auto-update can keep exact-version installs moving forward automatically
• migrate existing dynamic plugin entries to managed exact versions and roll config back if a background update install fails

Why

oh-my-openagent currently writes dynamic plugin entries such as @latest, which keeps startup coupled to OpenCode's plugin resolution path. When that path is slow or unstable, users can get multi-minute stalls before the plugin even loads. This change does not fix OpenCode core's startup fallback, but it does remove oh-my-openagent from relying on that path after install.

Testing

• bun test src/cli/config-manager/plugin-name-with-version.test.ts
• bun test src/cli/config-manager/plugin-detection.test.ts
• bun test src/hooks/zauc-mocks-bg/background-update-check.test.ts
• bun test src/hooks/zauc-mocks-ws/workspace-resolution.test.ts
• bun test src/hooks/auto-update-checker/hook.test.ts
• bun test src/hooks/zauc-mocks-hook/hook.test.ts
• bun test src/hooks/auto-update-checker/checker/pinned-version-updater.test.ts

Notes

• I intentionally left unrelated generated files (bun.lock, assets/oh-my-opencode.schema.json) out of this PR.
• There is an existing unrelated Windows path assertion issue in plugin-entry.test.ts; it is not introduced by this change.

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Summary by cubic

Switch plugin installs to managed exact versions with a persisted release channel. This removes @latest stalls at startup and lets auto‑update advance pinned installs safely.

• New Features

  • Installs oh-my-openagent@<version> via resolvePluginInstallReference, returning the exact entry and its channel.
  • Persists channel metadata in a config-scoped ...-plugin-state.json so updates track the right channel.
  • Background updater migrates dynamic/unpinned entries to exact versions, writes state, and reverts the config on failure.
  • Version bumps preserve the canonical alias (e.g., keep oh-my-openagent, migrate legacy oh-my-opencode safely).

• Migration

  • No manual steps. Existing dynamic entries are converted to exact versions during install or the next background check.
  • If an update fails, the config entry is rolled back automatically.

^{Written for commit a186d12. Summary will update on new commits. Review in cubic}

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