v1.13.15-openspec: reformat batch docs to OpenSpec directory structure

Adopt Fission-AI/OpenSpec's openspec/changes/<change-name>/{proposal,
specs,design,tasks}.md shape for BooCode's own batch docs. Zero-dep
documentation reformat; replaces ad-hoc boocode_batchN.md /
handoff_vN.N.N.md convention.

Existing batch docs moved into openspec/changes/archived/ via git mv
(preserves history):
- boocode_batch10.md
- handoff_v1.13.8_prefix_verify.md
- handoff_v1.13.10_per_tool_cost.md

Pre-v1.13.15 docs were NOT split into proposal/tasks/design files. The
work was already shipped; the originals are preserved as archived
snapshots. New v1.13.15+ batches land directly in
openspec/changes/<slug>/proposal.md (+ tasks.md, + design.md when
applicable) per the convention documented in openspec/README.md.

CLAUDE.md gained a one-line pointer to the convention (workflow
section). File grew from 153 → 154 lines, 27,682 → 27,925 chars; both
remain well under the AgentLint hard caps.

specs/ directory is reserved for future OpenSpec CLI adoption (v1.14+).
No CLI dep added in this batch — directory structure only. If/when the
full OpenSpec lifecycle is adopted, that lands as a separate batch.

CLAUDE.md

@@ -117,6 +117,7 @@ Required: `DATABASE_URL`, `LLAMA_SWAP_URL`. Optional: `PORT` (3000), `HOST` (0.0
 ## Workflow
 
 - Sam reviews all diffs and commits manually. Do not commit unless explicitly asked.
+- Per-batch docs live under `openspec/changes/<slug>/{proposal,tasks,design}.md`. Already-shipped batches are snapshots in `openspec/changes/archived/`. New batches follow the proposal+tasks shape; see `openspec/README.md` for the convention.
 - Deploy: `cd /opt/boocode && docker compose up --build -d` (or `docker compose build --no-cache boocode && docker compose up -d` if you suspect a layer-cache issue).
 - Git push to Gitea: `GIT_SSH_COMMAND="ssh -i /opt/boocode/secrets/boocode_gitea -o IdentitiesOnly=yes" git push origin <branch>`. The default agent identity is rejected; the in-repo deploy key (`secrets/`, gitignored) is the working one. Transient `Connection reset by peer` retries cleanly after `sleep 5`.
 - Don't accumulate `.bak-*` files. Clean them up in the same batch or immediately after merge.