keep the build queue in an uncommitted .scratch/

The queue is a working plan, not a record: git log already carries what landed on
each step, so committing the plan duplicates it and invites the two to disagree.
.scratch/ is gitignored and skipped by the gate the way ideas/ and reference/ are,
so exploratory files there need not compile.

state.md keeps a conditional pointer rather than a path it cannot promise: if the
scratch file is absent, the log is the plan.

docs/README.md

@@ -38,6 +38,7 @@ that is the finding to report.
 | language, translation, fallback | ADR-0004 + ADR-0009 |
 | deploy, containers, external services | ADR-0010 |
 | whether a future feature is worth building | `exploration.md` + `roadmap.md` |
+| what to build next | `.scratch/build-queue.md` if present, otherwise `git log` and `roadmap.md` |
 | how the harness works — a gate, counter, budget, the loop | `HARNESS.md` + `scripts/verify.sh` + `CLAUDE.md` |
 | a tool version, or why agent tooling stopped working | `toolchain.md` |
 | templates, layout, presentation, what a theme can rely on | `theme-contract.md` (+ ADR-0019, ADR-0023) |
@@ -81,6 +82,7 @@ Update in the Document step of the change that caused them. Not later.
 | New dependency approved | `scripts/allowed-deps.txt` + ADR |
 | Budget raised | `scripts/budgets.env` + ADR — once the first feature has shipped. While the harness is still being tuned, edit in place. |
 | A latent item fixed | remove from the Latent list in `state.md` |
+| A queue entry completed | tick it in `.scratch/build-queue.md` and note anything learned that changes a later entry — uncommitted, so the commit message carries what landed |
 | Arc finished | `roadmap.md` + a retro line in `state.md` |
 | Catalog item accepted or rejected | verdict line in `exploration.md` |
 | `CLAUDE.md`, `scripts/` or `.claude/` changed | `HARNESS.md`, same change. Enforced by `verify.sh`. |