Add read-only dedicated Goal pulse-check

[ByBrawe]

Fixes #161.

Adds a separate non-prompt-producing opencode-loopd pulse-check mode for dedicated Goals.

Behavior:

• reads .opencode/goals/*.json and reuses the dedicated-Goal continuation predicate;
• alerts only while the Goal is active;
• uses durable Goal updatedAt plus progress/revision changes as the stall authority;
• samples OpenCode session-list updated time only as corroborating telemetry;
• stores episode state only under .opencode/opencode-loop/pulse-check.json;
• never writes Goal state, dispatches a prompt, or runs /goal resume;
• emits at most one alert per stall episode and re-arms on Goal activity/status changes;
• supports one-shot execution and repeating --every checks, plus optional --goal <goal|session> targeting.

The shared state update is lock-protected so overlapping daemon/systemd one-shots cannot double-alert. Regression coverage proves one-alert-per-episode, waiting-user suppression, re-arming, read-only Goal state, no OpenCode run dispatch, and concurrent one-shot deduplication.

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