iter disc.1: boss-only commits + main-as-quarantine (no branches in implement)

Two project-wide rules are now explicit across every skill:

1. Only the Boss commits. No skill agent (implementer,
   brainstormer, planner, debugger, fieldtester, docwriter,
   architect, bencher) runs `git commit`. Agents write their
   artefacts to the working tree as unstaged changes; the Boss
   inspects, decides commit shape, and commits.
2. main HEAD is sacrosanct. No actor runs `git reset` or
   `git revert` on main. Bad work stays in the working tree
   where it is still discardable via `git checkout -- <paths>`.

Implement loses the `iter/<iter_id>` branch mechanic entirely;
Phase 0 of the orchestrator-agent now does a clean-tree check
and refuses to start on a dirty tree. Per-task agent commits
are removed everywhere; reviewers operate against
`git diff HEAD` instead of `pre_task_sha..head_sha`.

Motivation: 2026-05-11 iter 23.4 stranded prep2/prep3 commits on
an iter-branch that never integrated to main, then a corrected
spec falsely claimed those commits had shipped. Branch-per-iter
+ manual-Boss-merge + iter-stacking made the strand structurally
possible. See docs/journals/2026-05-11-iter-disc.1.md for the
full per-task notes and motivation.

docs/journals/INDEX.md

@@ -12,3 +12,4 @@
 - 2026-05-11 — iter cadence: audit/fieldtest/docwriter cadence restructure → 2026-05-11-iter-cadence.md
 - 2026-05-11 — iter 23.4-prep: checker prerequisites for prelude free fns → 2026-05-11-iter-23.4-prep.md
 - 2026-05-11 — iter gc.1: grounding-check agent + brainstorm Step 7.5 → 2026-05-11-iter-gc.1.md
+- 2026-05-11 — iter disc.1: boss-only commits + main-as-quarantine (no branches in implement) → 2026-05-11-iter-disc.1.md