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.

openspec/README.md

@@ -0,0 +1,38 @@
+# openspec
+
+Per-batch documentation convention adopted v1.13.15-openspec.
+
+Lift source: Fission-AI/OpenSpec directory layout. **No CLI dependency** — just
+the folder shape. Full OpenSpec lifecycle adoption is a future v1.14+ batch.
+
+## Layout
+
+```
+openspec/
+  changes/
+    <slug>/                          # one folder per shipped or planned batch
+      proposal.md                    # Why + scope summary
+      tasks.md                       # implementation step list
+      design.md                      # architecture / data-model decisions (optional)
+      specs/                         # reserved for future OpenSpec CLI adoption
+    archived/                        # snapshots of pre-v1.13.15 batch docs
+      <original-filename>.md
+  specs/                             # global specs, future v1.14+ use
+```
+
+## Conventions
+
+- Slugs are lowercase-hyphenated derived from the batch title
+  (e.g. `v1-13-10-per-tool-cost`, `file-attachments-v3-5`).
+- Already-shipped pre-v1.13.15 batches live in `changes/archived/` as
+  single-file snapshots. They were not split into proposal/tasks because
+  the work was already complete; archiving preserves git history.
+- New v1.13.15+ batches should land directly in
+  `changes/<slug>/proposal.md` (+ tasks.md, + design.md when applicable).
+- `proposal.md` carries the "Why" and scope. `tasks.md` is the action list
+  (numbered or checkbox). `design.md` is for non-trivial architectural
+  decisions worth recording separately.
+- A canonical dispatch brief (matching the v1.13.9 / v1.13.10 format)
+  is most naturally split as proposal.md (Where we are, Why this matters,
+  rationale sections) + tasks.md (Scope items, Build + smoke) + design.md
+  (Attribution model, Filtering, Canonical mapping).