docs: add context preservation invariants to CONSTITUTION

Add new invariants clarifying that:
- Archival is allowed, deletion is not
- Archive preserves Phase structure for traceability

Updated both the template and project's own CONSTITUTION.md.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: josealekhine

.context/CONSTITUTION.md

@@ -24,7 +24,7 @@ These rules are INVIOLABLE. If a task requires violating these, the task is wron
 
 ## TASKS.md Structure Invariants
 
-TASKS.md must remain a replayable checklist. Uncheck all items and re-run 
+TASKS.md must remain a replayable checklist. Uncheck all items and re-run
 the loop = verify/redo all tasks in order.
 
 - [ ] **Never move tasks** — tasks stay in their Phase section permanently
@@ -34,3 +34,8 @@ the loop = verify/redo all tasks in order.
 - [ ] **Use inline labels for status** — add `#in-progress` to task text, don't move it
 - [ ] **No "In Progress" / "Next Up" sections** — these encourage moving tasks
 - [ ] **Ask before restructuring** — if structure changes seem needed, ask the user first
+
+## Context Preservation Invariants
+
+- [ ] **Archival is allowed, deletion is not** — use `ctx tasks archive` to move completed tasks to `.context/archive/`, never delete context history
+- [ ] **Archive preserves structure** — archived tasks keep their Phase headers for traceability

.context/TASKS.md

@@ -59,7 +59,7 @@
 - [x] Implement `ctx tasks snapshot` — create point-in-time snapshot of TASKS.md
 - [x] Archive location: `.context/archive/tasks-YYYY-MM-DD.md`
 - [x] Keep Phase structure in archives for traceability
-- [ ] Update CONSTITUTION.md: archival is allowed, deletion is not
+- [x] Update CONSTITUTION.md: archival is allowed, deletion is not
 
 ### Phase 9: Claude Slash Commands (Skills) `#priority:medium` `#area:cli`
 - [ ] Research how existing skills are registered (check ralph-loop pattern)

internal/templates/CONSTITUTION.md

@@ -27,3 +27,8 @@ TASKS.md must remain a replayable checklist. Uncheck all items and re-run = veri
 - [ ] **Use inline labels for status** — add `#in-progress` to task text, don't move it
 - [ ] **No "In Progress" / "Next Up" sections** — these encourage moving tasks
 - [ ] **Ask before restructuring** — if structure changes seem needed, ask the user first
+
+## Context Preservation Invariants
+
+- [ ] **Archival is allowed, deletion is not** — use `ctx tasks archive` to move completed tasks to `.context/archive/`, never delete context history
+- [ ] **Archive preserves structure** — archived tasks keep their Phase headers for traceability