feat: multi-project engine runtime improvements

- Update project manager and runtime interfaces for multi-project coordination
- Add project engine configuration to core types and settings schema
- Enhance child-process worker tests for signal handling coverage
- Extend in-process runtime with per-project engine lifecycle hooks

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

packages/core/src/settings-schema.ts

@@ -36,6 +36,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   globalPause: false,
   enginePaused: false,
   maxConcurrent: 2,
+  globalMaxConcurrent: 4,
   maxWorktrees: 4,
   pollIntervalMs: 15000,
   groupOverlappingFiles: true,

packages/core/src/types.ts

@@ -902,6 +902,11 @@ export interface ProjectSettings {
   /** Maximum number of concurrent AI agents across all activity types
    *  (triage specification, task execution, and merge operations). */
   maxConcurrent: number;
+  /** System-wide maximum concurrent agents across ALL projects.
+   *  When multiple projects are active, the sum of their in-flight agents
+   *  will not exceed this limit. Applies to triage, execution, and merge.
+   *  Default: 4. When undefined, falls back to CentralCore default (4). */
+  globalMaxConcurrent?: number;
   maxWorktrees: number;
   pollIntervalMs: number;
   groupOverlappingFiles: boolean;