planning: add issue fix-work-agent-skip-active-locks to 2.1

fix work-agent to skip non-stale locked issues and find next available issue

Author: cowwoc

.cat/issues/v2/v2.1/fix-work-agent-skip-active-locks/index.json

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+{
+  "status" : "open"
+}

.cat/issues/v2/v2.1/fix-work-agent-skip-active-locks/plan.md

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+# Plan
+
+## Goal
+
+Update work-agent to skip non-stale locked issues and find the next available issue automatically,
+silently. work-prepare should expose a `stale: bool` field in its ERROR response (based on
+IssueLock.STALE_LOCK_THRESHOLD = 4 hours), so work-agent can decide whether to skip or offer cleanup
+without duplicating the threshold.
+
+When work-prepare returns ERROR with an existing worktree locked by another session:
+- If `stale == false` (lock_age_seconds < 14400): work-agent silently retries work-prepare to find
+  the next available issue, without presenting any AskUserQuestion dialog
+- If `stale == true` (lock_age_seconds >= 14400): work-agent preserves the existing cleanup-offer
+  behavior (Clean up and retry / Abort)
+
+## Pre-conditions
+
+(none)
+
+## Post-conditions
+
+- [ ] Bug fixed: work-agent no longer offers cleanup when encountering a non-stale locked issue
+- [ ] work-agent automatically retries work-prepare to find the next available issue when it receives
+      ERROR with `stale == false`, without presenting any AskUserQuestion dialog
+- [ ] work-prepare ERROR response (existing worktree locked by another session) includes a boolean
+      `stale` field, set to `true` when `lock_age_seconds >= 14400`, `false` otherwise
+- [ ] Staleness threshold sourced exclusively from `IssueLock.STALE_LOCK_THRESHOLD` in Java — no
+      hardcoded `14400` appears in skill Markdown code
+- [ ] Regression test added: work-agent skips non-stale locked issues and finds next available issue
+- [ ] Regression test added: work-agent preserves the cleanup-offer behavior when `stale == true`
+- [ ] No new issues introduced
+- [ ] E2E verification: reproduce the scenario (work-prepare returns ERROR with `stale == false`) and
+      confirm work-agent automatically finds the next available issue without any dialog