v0.8.0 — Agent Health & Dispatch Reliability

Agent health and dispatch reliability improvements, discovered during a
24-hour marketing team run where one agent was silently dead for 22 hours.

Fixes

• Fix preserve_working state desync after daemon restart — when a
  shim sends Event::Ready after respawn, only the shim handle's own
  state is used to decide whether to preserve Working. Previously the
  persisted daemon state (self.states) was also checked, causing freshly
  spawned agents to get permanently stuck as Working after a daemon restart.
  This was the root cause of priya-writer-1-1 being dead for 22+ hours.
• Dispatch queue prunes stale entries regardless of engineer state —
  process_dispatch_queue() now checks task validity (done/claimed/missing)
  before checking if the engineer is idle. Previously, entries for non-idle
  engineers were retained forever even when the underlying task was already
  completed by another engineer.
• Zero-output agent detection and auto-restart — agents with 0 output
  bytes after 10 minutes of uptime are now detected and cold-respawned.
  The health system previously had context pressure detection (too much
  output) and stall detection (no output change), but nothing to catch
  agents that never produced any output at all.