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>

sidecar/src/Eryx.AgentHost/Contracts/ProtocolModels.cs

@@ -187,8 +187,10 @@ public sealed class RunTurnMcpServerConfigDto
     public int? TimeoutMs { get; init; }
     public string? Command { get; init; }
     public IReadOnlyList<string>? Args { get; init; }
+    public IReadOnlyDictionary<string, string>? Env { get; init; }
     public string? Cwd { get; init; }
     public string? Url { get; init; }
+    public IReadOnlyDictionary<string, string>? Headers { get; init; }
 }
 
 public sealed class RunTurnLspProfileConfigDto

sidecar/src/Eryx.AgentHost/Services/SessionToolingBundle.cs

@@ -98,6 +98,7 @@ internal sealed class SessionToolingBundle : IAsyncDisposable
             Timeout = server.TimeoutMs,
             Command = server.Command,
             Args = server.Args?.ToList() ?? [],
+            Env = server.Env is null ? null : new Dictionary<string, string>(server.Env, StringComparer.Ordinal),
             Cwd = string.IsNullOrWhiteSpace(server.Cwd) ? null : server.Cwd,
             Tools = ResolveTools(server),
         };
@@ -116,6 +117,7 @@ internal sealed class SessionToolingBundle : IAsyncDisposable
             Type = server.Transport,
             Timeout = server.TimeoutMs,
             Url = server.Url,
+            Headers = server.Headers is null ? null : new Dictionary<string, string>(server.Headers, StringComparer.Ordinal),
             Tools = ResolveTools(server),
         };
     }