feat(tui): add delegated task workbench

[ZhiXiao-Lin]

Summary

• add /tasks and Ctrl+B as a live delegated-task control surface
• preserve semantic selection across bounded one-second refreshes, with search and full output preview
• route confirmed cancellation through Core while rejecting stale session, generation, and request results
• keep the parent turn streaming while the modal owns keyboard and mouse input

Validation

• cargo check --bin a3s
• cargo clippy --all-targets -- -D warnings
• 13 task-related binary tests
• slash command registry and policy audits
• cargo fmt --all
• git diff --check

[ZhiXiao-Lin]

Integrated through #40 at merge commit c252583e413f70a4cb265012d776f9d050b94de5. The consolidated v0.9.7 branch contains this PR's latest head b1813c5a90be8588b378e327423d7f2e3790bdd4 and passed Linux, macOS, and Windows CI.