Permission Profiles: named, switchable permission presets (global + per-repo)

### Describe the feature or problem you'd like to solve

I typically run with `/allow-all` for maximum productivity, but sometimes I want to restrict the agent (e.g., no shell execution, read-only mode, or limited directory access) for sensitive tasks or specific repos. Currently there's no way to:

1. Save a named permission configuration and switch to it quickly
2. Define per-repo permission defaults so certain repos always start restricted
3. Toggle between "full trust" and "restricted" without manually re-configuring each session

Related issues #3028 and #3050 cover parts of this (MCP tool trust and persistent directories), but neither addresses switchable named profiles.

### Proposed solution

Introduce **permission profiles** — named sets of permission rules that can be defined at two levels:

**Global profiles** (`~/.copilot/permission-profiles.json`):
```json
{
"profiles": {
"full-access": { "allowAll": true },
"read-only": {
"allowedTools": ["view", "grep", "glob", "github-mcp-server-*"],
"denyTools": ["powershell", "edit", "create"]
},
"no-shell": {
"allowAll": true,
"denyTools": ["powershell"]
}
},
"default": "full-access"
}
```

**Per-repo profiles** (`.github/copilot-permissions.json`):
```json
{
"default": "no-shell",
"trustedDirectories": ["."],
"profiles": {
"no-shell": {
"allowAll": true,
"denyTools": ["powershell"]
}
}
}
```

**Switching profiles in-session:**
- `/profile list` — show available profiles
- `/profile use ` — switch to a profile
- `/profile show` — show current active profile and its rules

Per-repo profiles would take precedence over global ones (team-enforced guardrails).

### Example prompts or workflows

1. Start session in infra repo → automatically applies "no-shell" profile (team policy)
2. Working on a PR review → `/profile use read-only` to ensure no accidental edits
3. Ready to implement → `/profile use full-access` to switch back
4. New session in any repo → applies user's global default without needing `/allow-all` each time

### Additional context

GitHub Copilot CLI 1.0.43

This would complement #3028 (granular MCP tool trust) and #3050 (persistent directory allow lists) by providing the overarching framework for managing permission sets.