feat(runtime): add managed child process registry

[luoye520ww]

Problem

The new managed-child contract needs a single owner for live child records before shell, LSP, Whisper, extension-host, and subagent integrations can share shutdown and cancellation handling.

Scope

This PR adds the in-memory registry on top of the contract from #903. It does not start, signal, or terminate OS processes.

Changes

• Register validated child records and reject duplicate IDs.
• Support owner-scoped listing and lifecycle updates for PID, detach state, and cleanup policy.
• Return defensive snapshots so callers cannot mutate registry state.
• Make remove and drain idempotent for repeated shutdown paths.

Safety

• Records are validated by the strict shared contract before insertion.
• No command, environment, token, path, or process output is stored.
• drain() only returns records for caller-owned cleanup; it never performs termination itself.

Typecheck

npm.cmd run typecheck

Passed.

Tests

npm.cmd exec vitest run src/main/services/managed-child-process-registry.test.ts
npm.cmd run lint
npm.cmd run build
git diff --check

Passed: 4 registry tests; root lint and build passed; diff check passed.

Review performed

• Functional correctness
• Registry ownership and lifecycle boundaries
• Idempotent cleanup behavior
• Data and secret safety
• Cross-platform contract compatibility
• Test quality and PR scope

Non-goals

• Process spawning or termination
• Shell, LSP, Whisper, extension-host, or subagent integration
• Persistence across application restarts

Dependency

Depends on #903 for ManagedChildProcess contract. After #903 merges, this branch should be rebased onto develop and the dependency commit dropped.

[luoye520ww]

Superseded by consolidated tool/process lifecycle PR #946, which passed Typecheck, Linux, macOS, and Windows packaged checks. Closing this duplicate to keep the review surface focused.