docs: sync contributor docs with the new issue and PR templates (#145)

## Linked issue

None — this is the nightly documentation sync. Labelled
`no-issue-needed`; the decisions being documented were already triaged
in the PRs below.

## Merged PRs covered

- **#144** — _docs: refresh CLAUDE.md (fix drift, add hard rules) and
add issue/PR templates_. Added `.github/ISSUE_TEMPLATE/` (bug report,
feature request, improvement; `blank_issues_enabled: false`) and
`.github/PULL_REQUEST_TEMPLATE.md`, and turned "use the templates" into
constitution §3. The developer docs still described the older,
template-less policy.
- **#142** — _chore: release v0.10.0_. No documentation surface: the
diff is `package.json` `version` plus a promoted `CHANGELOG.md` heading.
`docs/` states no version number and the build scripts read the version
from `package.json`, so nothing went stale.

## What changed in `docs/`

- `docs/development/README.md` — the "Before opening a pull request"
list now says an issue must be filed with one of the issue templates
(blank issues are disabled), and that the PR must follow the PR template
and its checklist.
- `docs/development/adding-a-command.md` — same correction in the
opening policy paragraph and in the final checklist item.
- `docs/development/project-structure.md` — the repository tree now
lists `.github/ISSUE_TEMPLATE/` and `.github/PULL_REQUEST_TEMPLATE.md`.

Nothing was added or deleted — no page became obsolete.

## Deliberately not documented

- The rest of #144's CLAUDE.md refresh (new constitution rules §6–§8,
corrected CI/release descriptions, module table). That file is the
constitution itself and `docs/` already links to it; restating it would
duplicate a source of truth. The CI/release facts it corrected already
matched `docs/development/build-and-release.md`, which was the accurate
one.
- #142 in full, per above.

## Checklist

- [x] Documentation-only change, entirely under `docs/`
- [x] Relative links in the touched files verified to resolve
- [x] Pre-commit hooks pass; commit is signed