feat: integrate Taskboard into Codex++ host

[acc-c]

Why this PR replaces #2296

PR #2296 was scoped as a standalone Taskboard application. The tested deliverable is the complete Codex++ integration: Taskboard embedded in the host, renderer injection, launcher/runtime routes, Manager entry points, Windows installer staging, and the corresponding CI. Keeping those changes in the standalone PR would make its description, review boundary, and tested artifact disagree, so #2296 is closed and this complete integration is submitted as one reviewable PR.

What changed

• Embed the Taskboard server and API in Codex++ with loopback as the default and explicit LAN opt-in.
• Add launcher/runtime routes, readiness handling, data-directory migration, project ownership validation, and Windows-safe child-process behavior.
• Add the Manager Taskboard menu/inline view and the renderer injector integration.
• Stage the Taskboard runtime in the Windows NSIS installer and split Manager vendor chunks.
• Add PR CI that checks the standalone Taskboard and integrated Windows/macOS artifacts.

Verification

• Local Taskboard typecheck, build, and tests: 396/396 passed.
• Local Manager check/build and tests: 255/255 passed with the machine's Git Bash path configured.
• Local cargo test --workspace: passed.
• Windows installer generated and staged runtime smoke-tested: /health 200 on 127.0.0.1, root page 200.
• Local and fork remote head: cc190d74bb521db6edeb32df178d43c1426d4064.

Generated installers and build directories are not committed.