feat(core): add TaskSupervisor for centralized async task lifecycle

[bug-ops]

Summary

Implements Phase 1 of the unified TaskSupervisor epic (#2958).

Introduces TaskSupervisor in zeph-core as a workspace-wide spawn surface for application-lifetime background tasks. The existing BackgroundSupervisor (per-turn agent loop) is unchanged — the two tiers serve different purposes and are not merged.

• Single spawn API: spawn(), spawn_blocking(), spawn_restartable()
• Automatic tracing: every task gets a span with task.name, task.origin, task.id, task.kind, task.restart_count; parent span captured from caller via tracing::Span::current()
• Panic capture: tokio::spawn boundary catches panics; reap driver protected from factory panics via catch_unwind(AssertUnwindSafe(...))
• Restart policies: RunOnce and Restart { max, delay } with correct lock release before sleep (no mutex held across await)
• BlockingError: { Panicked, SupervisorDropped } with proper JoinError::is_panic() mapping
• Graceful shutdown: CancellationToken-based with configurable timeout and task abort on expiry
• Task registry: list_tasks() returns Vec<TaskSnapshot> for TUI panel and CI diagnostics
• Performance: completion_tx moved to Inner to eliminate double mutex acquisition in spawn()

Also renames agent/supervisor.rs → agent/agent_supervisor.rs to remove naming ambiguity. Updates 8 import sites.

Test plan

• cargo build -p zeph-core — clean
• cargo nextest run -p zeph-core --lib -E 'test(task_supervisor)' — 10 tests pass
• cargo nextest run --workspace --lib --bins — all 7932 tests pass
• cargo clippy --workspace -- -D warnings — clean
• cargo +nightly fmt --check — clean

Related

Closes #2959 (Phase 1: Foundation)
Part of epic #2958

Next phases:

• feat(bootstrap): migrate runner.rs to TaskSupervisor — 7 memory loops, scheduler, skill watcher #2960 — Phase 2: migrate runner.rs + 7 memory loops
• feat(index,channels,a2a): migrate leaf crate spawns to TaskSupervisor #2961 — Phase 3: leaf crates (zeph-index, zeph-channels, zeph-a2a)
• feat(tui): task registry status panel using TaskSupervisor.list_tasks() #2962 — Phase 4: TUI task registry panel
• feat(core): TaskSupervisor observability — CPU-time metrics and tokio-console integration #2963 — Phase 5: CPU-time metrics + tokio-console