fm-afk-return.sh catch-up gate stays stuck on a torn-down task's status file forever #5

What

`.afk-return-catchup` (session state) records a lifecycle blocker per live task at the moment a
return catch-up opens: "status file unreadable: state/.status; catch-up stays gated". If that
task later finishes normally and is torn down (`bin/fm-teardown.sh`, which removes its `.status`
and `.meta`), the stale evidence line for it is never cleared - `fm-afk-return.sh check` keeps
reporting the same "status file unreadable" line for a task that no longer exists anywhere in the
backlog or state dir, forever, blocking every future away-mode re-entry.

Reproduced 2026-09-14 (the workstation (WSL))

• Away-mode return opened a catch-up gate that included `fm-image-cutouts-deliver` (in-flight at
  the time) among its lifecycle blockers.
• `fm-image-cutouts-deliver` finished normally minutes later (PR merged, task torn down - no
  `.status`/`.meta` file left anywhere).
• Roughly 40 minutes and many ordinary wakes later, attempting to re-enter away mode
  (`fm-afk-launch.sh propose`) refused with "return catch-up is still pending". `fm-afk-return.sh
  check` still listed the exact same stale line for the long-gone task, alongside a fresh, correct
  one for a task that was genuinely still starting up.
• Workaround used: deleted `state/.afk-return-catchup` entirely (private session-scoped state, safe
  to discard) and re-ran `check`, which rebuilt clean evidence from the currently-live task set and
  cleared immediately.

Suggested fix

`fm-afk-return.sh check` (or whatever owns `.afk-return-catchup`'s evidence lines) should drop a
lifecycle blocker for a task id that no longer has a `state/.meta` at all, rather than treating
"status file unreadable" as permanently unresolved. A torn-down task cannot ever produce a readable
status file again, so the current behavior makes the gate un-clearable without a manual file deletion
every time this sequence occurs (any away-mode session where a task finishes and tears down while
its return catch-up is still open).

Impact

Not destructive, but blocks all away-mode re-entry until manually worked around - would have kept
the owner from going properly afk tonight if the workaround hadn't been found.