fix(monitors): surface persistent poll failures instead of silent catch

[dngioidev]

Closes #318 (parent epic #183)

Problem

Both autopilot monitors wrapped each poll in catch { /* transient */ }, swallowing all errors forever. A persistent auth/network failure silenced the CI monitor permanently (the loop never learns CI state); a persistent fs error meant resolved decisions never surfaced, so parked/blocked tickets never unblock — a silent stall of the escalation-resume path.

Fix (thin, poll cadence preserved)

• New shared plugin/scripts/monitors/poll-guard.mjs — a pure consecutive-failure guard. Reset on any good poll; emit exactly one <monitor> error: <reason> (N consecutive polls) line once the streak reaches FAILURE_THRESHOLD (3); then throttle (REEMIT_EVERY = 10) so a lasting outage emits once per episode, never per poll. Keeps polling throughout.
• ci-watch.mjs: poll now returns ok/reason; distinguishes benign "no PR yet" (isNoPr) from a real gh failure. Tick applies the guard.
• decisions-watch.mjs: readDecisions rethrows non-ENOENT fs errors (ENOENT/missing dir stays benign); poll returns { lines, ok, reason }. Tick applies the guard.
• Healthy/quiet/transition behavior and the emit contract are unchanged.
• SKILL.md monitor section documents the new error line.

Acceptance criteria

• AC.1 — a monitor failing repeatedly (N consecutive polls) surfaces a one-line error to the loop instead of staying silent. Verified: AC-318.1 guard test asserts exactly one line at the threshold and that the counter keeps advancing (polling continues); AC-318.1 decisions test forces a real fs error (ENOTDIR) → ok:false.
• AC.2 — transient single failures do not spam; test covers transient-vs-persistent. Verified: AC-318.2 tests — a single/below-threshold failure surfaces nothing, and a success resets the counter so occasional blips never accumulate; ci poll marks a real gh failure ok:false but stays ok on no-PR.

Verification

• pnpm verify: 611/611 pass (monitors file 11/11, up from 4 — 7 new).
• No live-run impact (installed cache is separate from repo source).