HEARTBEAT.md cost reduction — collapse idle path to one bash block

FOLLOWUP to the May 19 structural rewrite (task 01d855fa). Safety goal achieved; cost goal not. This task targets the COST goal specifically with a properly-designed bar. WHAT TO DO: Restructure HEARTBEAT.md so the idle path is ONE bash block at the top of the document, doing both checks (active-ci-watch + fleet-tasks resume) and emitting structured stdout the model can branch on with a single if-then-else. Eliminate the numbered-step framing entirely for the idle path. Hypothesis: the model narrates every numbered step it executes, so removing the numbers removes the narration. Confirmed in 2026-05-20 LEARNINGS analysis of 67 sessions: 49 of 67 had exactly 4 tool turns (one per numbered step that has a bash block). SUCCESS BAR (TWO METRICS, BOTH MUST HOLD, MEASURE OVER 50-SESSION OVERNIGHT WINDOW POST-EDIT): - COST: median tool-call rounds per idle heartbeat <= 2 (currently 4) - SAFETY (co-bar, MUST NOT regress): zero unsolicited DMs to Stefan AND >= 95% of pure heartbeat sessions end with HEARTBEAT_OK as the final assistant text (currently 97%) If cost passes but safety regresses, the fix is bad and rolls back. If safety holds but cost does not move, iterate. MEASUREMENT SCRIPT: use the Python block from 2026-05-20 reflection (in /home/agent/agents/stefan-engineer/memory/2026-05-20.md). Tool-turn distribution per heartbeat + count of sessions ending with HEARTBEAT_OK + count of outbound message action:send across the window. Self-contained, runs against ~/.openclaw/agents/stefan-engineer/sessions/. TWO-PHASE CLOSURE (carry-over from May 19 process improvement): - Phase 1: edit committed to HEARTBEAT.md. - Phase 2: next-cycle overnight measurement against the two-metric bar above. Task remains in_progress overnight after phase 1; next morning reflection either completes it (both bars pass) or PATCHes the failure data into description and iterates. DEFER RULE (per May 18 LEARNINGS): batch the edit with the next legitimate coding session that requires workspace context. 7-day cap: if no such session arrives by 2026-05-27, drop the batching requirement and land it standalone. This is NOT a defer-as-drift case because the safety metric is solidly green; this is pure cost optimization with a measured baseline. NOTE TO FUTURE-ME: do NOT add another append to HEARTBEAT.md. Read the whole file top-to-bottom, identify which sections collapse into the new top-of-file idle-path bash block, and rewrite cohesively. The previous append-only approach is what got us into the structural-conflict mess.