feat(power): keep the Mac awake while agents are running

[Legacynnn]

What

Prevent macOS idle system sleep while ≥1 agent or terminal stream is active, so long-running turns keep going when the user steps away (lid open) instead of dying on a heartbeat timeout.

How

• New platform/power.rs: a SleepGuard ref-counter backed by a swappable SleepAssertionBackend trait — real IoKitBackend (IOPMAssertionCreateWithName/Release, PreventUserIdleSystemSleep) on macOS, NoopBackend elsewhere.
• ActiveStreams (the single source of truth for "is anything running") owns one SleepGuard and drives on_count_change off its map size on every register/unregister, under the existing map mutex. First stream acquires the assertion; last one out releases it. Terminal sessions participate via set_session_active.
• Best-effort: a failed assertion is logged and ignored, never blocking a send. macOS auto-releases a process's assertions on exit, so a crash can't strand the Mac awake.

Scope

Idle system sleep only — the display may still sleep (battery friendly), and closing the lid on battery still sleeps as usual (SMC-enforced; out of scope). No schema/pipeline/frontend change.

Verification

• Unit + ref-count tests (fake backend): acquire-once-for-concurrent-streams / release-on-last, rejected-duplicate is a no-op, terminal sessions participate.
• Full cargo test green; cargo clippy --all-targets -- -D warnings clean.
• Real-IOKit check: acquiring makes a PreventUserIdleSystemSleep "Helmor is running an agent" line appear in pmset -g assertions, and releasing clears it.