feat: auto-discover MCP servers from project and user config files

Scan .vscode/mcp.json, .mcp.json, .copilot/mcp.json (project-level) and
~/.copilot/mcp.json (user-level) for MCP server definitions. Discovered
servers require explicit user acceptance before activation.

Backend:
- Add ConfigScannerRegistry with per-format scanners and  substitution
- Add discovered tooling domain model with fingerprint-based change detection
- Integrate scanning into workspace load, project add, and project selection
- Merge accepted discovered MCPs into effective runtime tooling
- Extend sidecar contracts with env/headers for real-world MCP configs
- Add IPC channels for accept/dismiss/rescan operations

Frontend:
- Add DiscoveredToolingModal for reviewing pending MCP servers
- Auto-show modal when pending discoveries exist on load or project switch
- Add amber badge on sidebar project headers for pending discoveries
- Add discovered MCP section in Settings panel with accept/dismiss/rescan
- Group InlinePills tool dropdown by Workspace MCP / User MCP / Project MCP
- Pass effective project tooling (including accepted discoveries) to ChatPane

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/contracts/channels.ts

@@ -4,7 +4,10 @@ export const ipcChannels = {
   loadWorkspace: 'workspace:load',
   addProject: 'workspace:add-project',
   removeProject: 'workspace:remove-project',
+  resolveWorkspaceDiscoveredTooling: 'workspace:resolve-discovered-tooling',
   refreshProjectGitContext: 'projects:refresh-git-context',
+  rescanProjectConfigs: 'project:rescan-configs',
+  resolveProjectDiscoveredTooling: 'project:resolve-discovered-tooling',
   savePattern: 'patterns:save',
   deletePattern: 'patterns:delete',
   setPatternFavorite: 'patterns:set-favorite',