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>

tests/main/sessionToolingConfig.test.ts

@@ -17,10 +17,21 @@ const TOOLING: WorkspaceToolingSettings = {
       command: 'node',
       args: ['server.js'],
       cwd: 'C:\\workspace\\repo',
+      env: { DEBUG: 'true' },
       tools: ['git.status'],
       createdAt: TIMESTAMP,
       updatedAt: TIMESTAMP,
     },
+    {
+      id: 'mcp-remote',
+      name: 'Remote MCP',
+      transport: 'http',
+      url: 'https://example.com/mcp',
+      headers: { Authorization: 'Bearer token' },
+      tools: ['remote.tool'],
+      createdAt: TIMESTAMP,
+      updatedAt: TIMESTAMP,
+    },
   ],
   lspProfiles: [
     {
@@ -61,7 +72,7 @@ describe('session tooling config helpers', () => {
   test('builds a run-turn tooling config from selected MCP and LSP ids', () => {
     expect(
       buildRunTurnToolingConfig(TOOLING, {
-        enabledMcpServerIds: ['mcp-git'],
+        enabledMcpServerIds: ['mcp-git', 'mcp-remote'],
         enabledLspProfileIds: ['lsp-ts'],
       }),
     ).toEqual({
@@ -74,6 +85,15 @@ describe('session tooling config helpers', () => {
           command: 'node',
           args: ['server.js'],
           cwd: 'C:\\workspace\\repo',
+          env: { DEBUG: 'true' },
+        },
+        {
+          id: 'mcp-remote',
+          name: 'Remote MCP',
+          transport: 'http',
+          tools: ['remote.tool'],
+          url: 'https://example.com/mcp',
+          headers: { Authorization: 'Bearer token' },
         },
       ],
       lspProfiles: [