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/phase-template.md

@@ -11,13 +11,13 @@
 {{or "— (none)"}}
 
 ## Tasks
-1. `{{path/to/file}}` — {{specific change, file-level detail}}
-2. `{{path/to/file}}` — {{specific change}}
+1. `01_{{short_name}}.md` — {{one-line summary}}
+2. `02_{{short_name}}.md` — {{one-line summary}}
 
 ## Testing & Quality
 - Unit/integration: {{tests required for all new logic — name the behaviors to cover}}
 - Coverage: **>90%** on new/modified code
-{{project additions, e.g. a dedicated Playwright E2E suite run in isolation}}
+{{project additions, e.g. a dedicated Playwright E2E suite run in isolation by this phase's final pass}}
 
 ## Completion Criteria
 - [ ] {{observable check: command to run / endpoint to hit / artifact to exist}}