[Feature Request] Add /diff panel exclusions, default base mode setting, and document composerSidebarBackground token

**Bug Description**
[FEATURE] /diff panel: path exclusions, a global default base mode, and a documented background token
1. **Exclude paths from the panel.** Runtime-state files in dotfiles repos (e.g. `.claude.json.*`, shell history, lockfiles like `flake.lock`) dominate the panel. The only current way to hide them is a `Read(...)` rule in `permissions.deny`, which also blocks Claude from reading them. `linguist-generated` in `.gitattributes` isn't honoured, and the built-in noise filter doesn't catch `flake.lock`. Please add a settings key, e.g. `diffPanel.exclude: ["**/.claude.json.*", "**/flake.lock"]`.
2. Global default base mode. Ctrl+X B is saved per project, so every new folder starts on session. Please add a settings key, e.g. diffPanel.defaultBaseMode: "uncommitted".
3. Pre-session untracked files don't appear in the default view, which isn't obvious. A note in the panel would help.
4. Document the panel background as a theme token. The panel paints its own background from composerSidebarBackground (rgb(38, 38, 38) in the dark theme). That looks like a grey box on terminals with a darker background. Overriding it in a custom theme (~/.claude/themes/*.json) works, but it's missing from the color token reference in the terminal-config docs, so it could change without notice. Please list it with the other fullscreen tokens.

Workarounds in use: Read(...) deny rules for exclusions, and a custom theme overriding composerSidebarBackground. Both rely on undocumented behaviour.

Related: #86944, #94121

**Environment Info**
- Platform: darwin
- Terminal: fullscreen TUI (tmux in Ghostty)
- Version: 2.1.270
- Feedback ID: 27044a4d-4a90-4e80-9b55-85838438260c

**Errors**
```json
[]
```

Start by locating the /diff panel settings and the terminal-config color-token reference; the issue names no source files or tests. Confirm how exclusions, the global default base mode, untracked-file guidance, and composerSidebarBackground are currently handled, then verify each requested setting or documentation change in the fullscreen panel and terminal-config docs.

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