install-hooks: add --no-editor-extensions flag and disable_editor_extensions config to skip IDE extension installs

[ashwin-corridor]

Motivation

git-ai install-hooks unconditionally attempts to install the IDE extensions/plugins (git-ai.git-ai-vscode into VS Code/Cursor/Windsurf, plus the JetBrains plugin) as part of the same step that installs the agent hooks. Today there is no way to get the agent hooks without the editor extensions: install-hooks has no opt-out flag and no config key, and since hooks are re-checked/re-installed daily (and the auto-update path re-runs the installer), anything removed out-of-band gets re-applied.

For managed/enterprise rollouts it's often desirable to deploy the binary and agent hooks while leaving editor extensions to each organization's own extension management. This follows the existing precedent of opt-in flags on install-hooks (--skills, --visual-studio-extension).

Changes

• New install-hooks --no-editor-extensions flag. Skips IDE extension/plugin installs (VS Code, Cursor, Windsurf, JetBrains, and Visual Studio when opted in) while still installing all agent hooks (Cursor hooks.json, Claude Code, Codex, Gemini, etc.) — and, for VS Code, still configuring the chat-hook settings, since those drive agent attribution.
• New disable_editor_extensions config key (git-ai config set disable_editor_extensions true). install-hooks honors it when run without flags, so the daemon's daily hook refresh and the auto-update/install-script path respect the opt-out. Supported by config get/set/unset/show-all and documented in config --help.
• Plumbed as a new skip_editor_extensions field on HookInstallerParams; each extension-installing installer returns a non-changing "skipped" result instead of attempting the install.
• Help text and a README FAQ entry ("Can I install the agent hooks without the IDE extensions?").

Defaults are unchanged: extensions still install unless explicitly opted out. uninstall-hooks is unaffected.

Testing

• cargo test --lib: 2043 passed.
• cargo clippy --all-targets -- -D warnings (1.93.0) and cargo fmt --check: clean.
• Integration tests for config_cli (incl. the FileConfig CLI-coverage guards, which now cover the new key) and install_hooks: pass.
• Manual end-to-end with a stub cursor CLI on PATH:
  • default dry-run: Cursor: Pending updates + Cursor: Pending extension install
  • with --no-editor-extensions: Cursor: Pending updates only — hooks still installed, extension skipped
  • config set disable_editor_extensions true then a plain install-hooks: extension skipped with no flag; config unset restores default behavior.
• New unit tests cover flag parsing and the skip path for the Cursor, VS Code, Windsurf, JetBrains, and Visual Studio installers.

Design notes

• --no-editor-extensions covers the JetBrains plugin and Visual Studio extension too, not just the VS Code-family extension.
• VS Code's chat-hook settings (agent attribution for Copilot chat) are intentionally still written when extensions are skipped; that's hook wiring, not an extension.
• The config key is what makes the daemon's daily re-install respect the opt-out (a flag alone would be undone by the flag-less daily refresh).