feat: phases + tasks — per-task execution with phase directories

- A phase is now a directory: 00_phase.md (overview + task index) plus
  small, quick NN_task.md task files; complete/ mirrors todo/
- Task is the unit of execution: run-task.sh (one unit), run-phase.sh
  (phase to completion incl. 00_phase.md final pass), auto-phase.sh
  (all units in order); validate.sh gate runs after every task
- PHASE_COMMIT commits at phase boundaries (00_phase.md / legacy file moves)
- Legacy flat todo/NN_name.md files still execute as a single unit;
  new migrate-phases-to-tasks.sh converts them to the directory layout
- phase-status.sh reports per-task state and the next unit
- New task-template.md; phase templates now carry a Tasks index

phase-authoring/assets/task-template.md

@@ -0,0 +1,19 @@
+# Task {{NN}} — {{Short Title}}
+
+**Phase:** `{{NN_phase}}` · **Story:** `{{.agent/user_stories/<story>.md or "n/a"}}`
+
+## Objective
+{{1–2 sentences: what this task delivers}}
+
+## Work
+1. `{{path/to/file}}` — {{specific change, file-level detail}}
+2. `{{path/to/file}}` — {{specific change}}
+
+## Testing & Quality
+- Unit/integration: {{tests required for this task's logic}}
+- Coverage: **>90%** on this task's new/modified code
+
+## Completion Criteria
+- [ ] {{observable check: command to run / endpoint to hit / artifact to exist}}
+- [ ] full test suite green
+- [ ] no behavior change in completed work