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/main/sessionToolingConfig.ts

@@ -55,6 +55,7 @@ export function buildRunTurnToolingConfig(
           command: server.command,
           args: [...server.args],
           cwd: server.cwd,
+          env: server.env ? { ...server.env } : undefined,
         },
       ];
     }
@@ -63,12 +64,13 @@ export function buildRunTurnToolingConfig(
       {
         id: server.id,
         name: server.name,
-        transport: server.transport,
-        tools: [...server.tools],
-        timeoutMs: server.timeoutMs,
-        url: server.url,
-      },
-    ];
+          transport: server.transport,
+          tools: [...server.tools],
+          timeoutMs: server.timeoutMs,
+          url: server.url,
+          headers: server.headers ? { ...server.headers } : undefined,
+        },
+      ];
   });
 
   const lspProfiles = selection.enabledLspProfileIds.flatMap((id): RunTurnLspProfileConfig[] => {