v0.7.4 — progress.md heartbeat: one live line per stage

Heartbeat (AGENTS.md rule 9)

.sdlc/work/<slug>/progress.md now holds exactly one overwritten line —
<stage>[ n/m] · <what is happening> · <ISO timestamp> — so a human can see
where the loop is at any moment, in every stage, not just at gates.

• Every stage writes it: on entry, at every sub-task change, before each
  dispatch; build counts plan.md's Order of work as build n/m and updates
  per fix-loop round.
• Long sub-tasks refresh it at natural checkpoints (a command finished,
  a file edited) even when the text does not change — a stale heartbeat means
  a dead loop, not a slow step.
• status.sh shows it as a now → line with its age (BSD/GNU stat);
  older than 30 min gets — STALE. Absent file: silent, fully
  backward-compatible.
• A signal, not a record: init.sh gitignores it under work/ and
  archive/; history stays in deviations.md / evidence.md / harvest.md.
  Follow it live with watch -n5 cat .sdlc/work/<slug>/progress.md.

Also retro-tagged v0.7.3 (optional qa: tool in config, intent Refs: line).