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

phased-execution/assets/executor-prompt.md

@@ -5,7 +5,7 @@ Target phase file: `.agent/phases/todo/{{PHASE}}`
 ## Steps
 1. Read `.agent/PLAN.md` — project goals, architecture, and **LOCKED DECISIONS** (binding; never introduce technology outside them).
 2. Read `AGENTS.md` if present.
-3. Read every file in `.agent/phases/complete/` so your work stays architecturally consistent with what is already built.
+3. Read the completed phase files so your work stays architecturally consistent with what is already built: every `00_phase.md` in `.agent/phases/complete/*/`, plus any legacy flat phase files directly in `.agent/phases/complete/`.
 4. Read the target phase file and complete **every** task in it, in order.
 5. Write the unit and integration tests required by the phase's Testing & Quality section. Do not omit parts of the code to inflate coverage.
 6. Run the project's full test suite and linter. If anything fails — including the phase's coverage criterion — fix it and re-run until green.