Proposal: user-local and global config layers so personal settings are not committed #296

Problem

All UI settings except translation engines are persisted to openspec/.openspecui.json inside the project. As soon as I change the theme, editor, terminal or notification preferences, the file shows up in git status and is offered for commit. These are personal preferences: my teammates use different ones, so committing them either overrides their setup or creates constant churn in PRs.

Today the only workarounds are .git/info/exclude, git update-index --skip-worktree or a symlink, and none of them lets me keep the same personal settings across projects.

Current behavior (as of 4d8c007)

• ConfigManager (packages/core/src/config.ts) reads and writes only <project>/openspec/.openspecui.json. The path is hardcoded, there is no CLI flag or env var to relocate it.
• GlobalSettingsManager (packages/core/src/global-settings.ts) writes ~/.openspecui/settings.json, but its schema covers only translation, translationCache, translationEngines and servePresentation.
• There is no merge or fallback between the two files.

Proposal

Introduce layered resolution for OpenSpecUIConfig, lowest to highest priority:

1. Built-in defaults
2. User global config: ~/.openspecui/config.json (respecting OPENSPECUI_HOME), same schema as OpenSpecUIConfig
3. Project shared config: openspec/.openspecui.json (committed, team-wide, unchanged semantics)
4. Project local config: openspec/.openspecui.local.json (personal, never committed)

Details:

• Deep merge per key, the same way the current partial-override persistence already works.
• The Settings UI lets the user pick the write scope: "All my projects" (global), "This project, only me" (local), "This project, shared with team" (shared). The default scope for new changes should be local or global, so personal changes do not touch the committed file unless explicitly requested.
• Each setting shows where its effective value comes from, with a reset per layer.
• On first write to .openspecui.local.json, add it to .gitignore (or at least document it and warn if it is not ignored).
• All three files are watched reactively, as openspec/.openspecui.json is today.
• Backward compatible: an existing openspec/.openspecui.json keeps working as the shared layer, no migration required.

Optionally, an env var such as OPENSPECUI_CONFIG to point the global layer to a custom file.

Acceptance criteria

• Changing personal settings in the UI with the default scope does not modify any tracked file.
• The same personal settings apply in every project without copying files.
• Team-wide values (for example cli.command / cli.args) can still be committed in openspec/.openspecui.json.
• README documents the layers, file paths and precedence.