feat: tick in-body checkboxes from a domain script

A checkbox is the one part of a body that is state and not prose.
Everything else is written once; boxes get ticked as the work goes, and
until now the only ways to tick one were a human with an editor or a
model rewriting the whole body. The second is worse: the rewrite re-flows
lines and re-words sentences, so the issue's diff swells around a change
that means one character. Progress was invisible too — issue_index.py
builds INDEX.md from metadata and never looked inside a body, so "3 of 7
done" required opening the file.

All three pieces are domain: a checkbox is body syntax, which is part of
the answer to "what is an issue". The parser goes in issue.py so the sync
layer can reuse it instead of redefining the format on its own side.

issue.py gains checkboxes(text) -> [Checkbox(index, line, end_line,
checked, text, section)], plus set_checkbox(text, item, checked) and
checkbox_progress(text). All pure, no I/O, importable from another layer.
The scan covers the whole text, in any section: the type/feature template
keeps child issues as checkboxes under `## Issues`, so binding the parser
to `## Acceptance criteria` would silently lose half of them; the heading
is recorded, never required. Only a marker line opens an item, so a
wrapped continuation line belongs to the item above it rather than
counting as one of its own. A `- [ ]` inside a code fence is an example
of the markup and is skipped. Line numbers are relative to the text
given, which is what lets a caller work on a body or on a whole file.

issue_ac.py lists the items numbered, grouped by heading, and ticks one
by number or by substring. An ambiguous substring is an error that prints
the matches — a coin flip would tick the wrong box and look like it
worked. It patches the file rather than round-tripping through
Issue.to_text(), so exactly one character changes: metadata order,
wording, wrapping, trailing whitespace and CRLF endings all come back
byte for byte, proven by a diff in the tests.

INDEX.md gains a progress column: `3/7` for an issue with checkboxes,
blank for one without. Counted off the body at build time and stored in
no field — a second copy of the state would be wrong by the next edit.

issue_check.py is unchanged and stays that way on purpose: an unticked
box is work not done yet, not a malformed issue, and validate() carries a
comment saying so.

Delivering a tick to the tracker is out of scope — that is push.py
--update in /tea:sync.

format.md gets one clarifying bullet. It said acceptance criteria are
checkboxes but never said what a checkbox is, so the parser had to settle
questions the format left open: any section, wrapped items, fenced
examples. Those rules are now written down where the parser and the sync
layer can both point at them.

tests/ is new, and is the convention: plain stdlib unittest, no pytest
and no third-party deps, since the code under test may not have
dependencies either. Scripts are imported via sys.path.insert and every
fixture is built in a TemporaryDirectory, never in tmp/.

    python3 -m unittest discover -s tests -v     32 tests, OK

skills/issue/scripts/ still imports stdlib only, with no subprocess.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

agents/tea-runner.md

@@ -30,7 +30,8 @@ to fill the gap yourself.
 Load the skill, do not remember the flags:
 
 - `/tea:sync` — `pull.py`, `push.py`, `comment.py`, `remote.py`, `labels.py`
-- `/tea:issue` — `issue_check.py`, `issue_tree.py`, `issue_index.py`, `issue_new.py`
+- `/tea:issue` — `issue_check.py`, `issue_tree.py`, `issue_index.py`,
+  `issue_new.py`, `issue_ac.py`
 
 Invoke `Skill` with `tea:sync` or `tea:issue` at the start of the task, and use
 the command table it gives you verbatim. The skill is the single source of
@@ -46,7 +47,11 @@ instead of trying it.
    the `tea-guard` hook substitutes the pinned login. Never name a login.
 2. **No writing to issue files.** You have no `Edit` and no `Write`. Scripts
    write files; you do not. If a task needs a body edited or a metadata field
-   changed by hand, stop and say which file and which field.
+   changed by hand, stop and say which file and which field. `issue_ac.py` is
+   the one script that touches a body, and it changes a single character: tick
+   only the items the caller named, by the number or the substring the caller
+   gave. Whether a criterion is actually met is a judgement about content, and
+   content is never yours.
 3. **Push only what you were told to push.** `push.py` publishes to a tracker
    other people read. Run it with the ids the caller named, or with the filter
    the caller named. Never widen the set, never run a bare `push.py` because it