chore(log): the handoff log was serialising every parallel branch

DECISIONS-LOG.md was append-only in ONE file and every agent appended to its
tail, so any two concurrent branches collided on the same last hunk. Measured
against the live Gitea today: four of the five open PRs were mergeable=false,
and `git merge-tree` named DECISIONS-LOG.md as the ONLY conflicting path in
all four.

    #22 fix/crafty-oauth2-proxy-signin       CONFLICT: DECISIONS-LOG.md
    #20 feat/conformance-job-template        CONFLICT: DECISIONS-LOG.md
    #19 feat/conformance-trigger-kubeconfig  CONFLICT: DECISIONS-LOG.md
    #18 feat/conformance-runner-image        CONFLICT: DECISIONS-LOG.md
    #17 feat/ci-is-the-approver              clean — it wrote no handoff

Nothing was wrong with any of those changes. The log's storage format
serialised work that was otherwise parallel, and the cost grew with every
concurrent branch (N branches -> N-1 rebases, each invalidating the gate run
before it). The agents work in isolated worktrees precisely so they do not
block each other; this one file undid that.

So: stop sharing a file. Handoffs are now one-per-file under decisions/,
named YYYY-MM-DD-<slug>.md. Disjoint paths have nothing to merge. There is
deliberately no index file (a shared mutable file is the thing that just
failed) and no counter (two agents would race for the same number) — the
directory listing is the index.

DECISIONS-LOG.md is frozen as the pre-2026-09-29 archive, its SHA-256 pinned
in decisions/ARCHIVE.sha256, and a new gate stage rejects any change to it.
That is a guard and not a paragraph because the old habit is written into the
runbooks and into every agent that has read the log, and getting it wrong does
not fail: it produces a clean-looking commit that re-creates the conflict on
OTHER branches, later, misattributed.

  ci/decisions-log.sh        new stage: archive pinned; every entry named with
                             a real date, carrying date/branch/title frontmatter
                             that agrees with its filename, and all three of
                             CLAUDE.md §7's sections with a real body under each.
                             Exit 0 or 1 only — no skip path by construction, so
                             no fail-open surface on a guard that closes one.
  ci/gate.sh                 runs it; fail and (impossible) skip both BLOCK.
                             No REQUIRE_DECISIONS_LOG knob: a guard with an off
                             switch is not a guard.
  ci/tests/decisions-log.test.sh   22 assertions, incl. the one that matters —
                             an append to the archive is rejected, and the
                             message names the file to write instead.
  ci/tests/gate-decision.test.sh   +3 assertions for the new wiring, AND a
                             pre-existing bug: the stub list never included
                             gitea-settings, so run_stage saw exit 127 and
                             recorded a FAILED stage in every case. 15 of 47
                             assertions failed on main for that reason alone —
                             the suite was measuring its own missing stub.
                             Now 50/50.
  CLAUDE.md                  §7 rewritten; §2b widened to the new path.
  ci/Makefile, ci/README.md, .gitea/workflows/README.md — stage named where
                             the other stages are named.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

ci/README.md

@@ -101,6 +101,7 @@ caller that must distinguish them invokes `./ci/gate.sh` directly.
 |---|---|---|
 | `lint`, `l0`, `render-golden`, `l1` | a **pinned tool is absent** or a **wired seam went unwired** — it *could not run* | **BLOCKED** under strict skips (a CI runner: the toolchain is installed for us by `.gitea/workflows/scripts/install-tools.sh`, so a skip is a defect); **UNDETERMINED** otherwise (a workstation missing a tool is a real local condition — still non-zero, still not a pass; the remedy is `infra/devcontainer/dev.sh`, which carries the pinned toolchain) |
 | `spec-immutability` (R-TEST-06) | the sibling spec repo is not checked out, so the spec **could not be verified** | **UNDETERMINED** — unverified is unverified. Deliberately *not* upgraded to BLOCKED by strict skips: the live runner does not check the spec repo out at all, so blocking would leave the only enforced status check permanently red for a reason no PR could clear. It is therefore the **one** skip whose non-answer a caller may knowingly tolerate, and `.gitea/workflows/gate.yml` does so in one visible line. `REQUIRE_SPEC_VERIFICATION=true` makes it a hard block once that checkout is wired. Locally: check the spec repo out beside this one, or set `SPEC_REPO` (`dev.sh` mounts a sibling automatically). |
+| `decisions-log` | **nothing** — this stage has no skip path. It reads only files in the tree under test and needs no tool and no cluster, so there is no environment in which it "could not run" | a reported skip is a **defect in the stage** and BLOCKS. The stage fails when the frozen `DECISIONS-LOG.md` is modified (which silently re-creates, for every *other* open branch, the tail-hunk conflict that made four of five PRs unmergeable on 2026-09-29) or when a `decisions/` entry is malformed. There is deliberately **no** `REQUIRE_DECISIONS_LOG` knob: a guard with an off switch is not a guard. See `decisions/README.md`. |
 | `conformance` (L2/L3) | `CLUSTER_AVAILABLE!=true` — the environment **declared** the capability absent (documented Phase-0 behaviour) | **does not deny a PASS**: a declared-inapplicable stage was never *supposed* to run. It is NAMED in the PASS headline and step summary, the touched requirements it would have covered are reported UNVERIFIED, and `REQUIRE_CLUSTER_TESTS=true` (Phase 1+) turns that into a block. A skip **despite** `CLUSTER_AVAILABLE=true` is a defect and BLOCKS. |
 
 That asymmetry is the whole rule: **declared-not-applicable may pass; could-not-run may not.**