fix: resolve the issue store path independently of the working directory

ISSUE_ROOT was the relative `tmp/issues`, so "the store" was whatever
directory the shell happened to be standing in. It is the --out default
in all eight scripts of both layers, which made one `cd` — and a `cd`
outlives the command that ran it — enough for readers to report an empty
store on a full one and for writers to quietly build a second store
beside the first. `issue_index.py` run from inside tmp/issues left
tmp/issues/tmp/issues/ behind and exited 0.

The anchor is issue.py's own __file__, not cwd. A script's location is a
fact about the installation; cwd is a fact about the last `cd`, and the
scripts are invoked by path from wherever the agent happens to be. From
there `store_root()` walks up to the nearest repo marker — `.git`
(exists(), not isdir(): a worktree's .git is a file) or AGENTS.md for a
copy taken out of git — and joins tmp/issues. Markers rather than a
fixed number of `..` hops, because the layout is not a promise. cwd is
tried only if the scripts are not inside a repository at all.

The function lives in the domain layer and skills/sync imports it, so
both layers agree by construction — the direction the layering rule
allows. skills/issue stays stdlib-only.

An explicit --out still wins and is used exactly as typed: a relative
--out stays relative to cwd, because that is what the operator asked
for. No new environment surface.

Two consequences the issue also asked for:

- Missing is no longer reported as empty. `store_error()` returns one
  message for a path that is not there and another for a store with no
  issues in it.
- Nothing conjures a store as a side effect of a write. save() and
  issue_index.build() require it instead of os.makedirs'ing it; only
  issue_new.py and pull.py create one, and both say so on stderr.

Establishes tests/ — plain stdlib unittest, no pytest, no dependencies.
The store tests build a throwaway repo in a TemporaryDirectory (a .git
marker, a copy of both script layers, fixture issues) and run the real
scripts inside it as subprocesses from five different working
directories; tmp/issues/ is never touched. Against the pre-fix scripts
15 of the 21 fail, reproducing the report exactly — five stray stores,
including tmp/issues/tmp/issues.

    python3 -m unittest discover -s tests -v

Closes claude-skills/tea#15

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

AGENTS.md

@@ -68,6 +68,25 @@ the domain layer, it is in the wrong place.
 - `hooks/` — PreToolUse hooks: `tea-guard` blocks or rewrites `tea` invocations
   that don't use the pinned login; `agents-sync` keeps every directory canonical
   (`AGENTS.md` real file, `CLAUDE.md` symlink to it)
+- `tests/` — stdlib `unittest`, no third-party anything
+
+## Tests
+
+```bash
+python3 -m unittest discover -s tests -v
+```
+
+Plain `unittest`; no pytest, no dependencies — the scripts under test are
+stdlib-only and the tests hold the same line. `skills/*/scripts/` are not
+packages, so a test that needs the domain module imports it with
+`sys.path.insert`.
+
+**A test never touches `tmp/issues/`.** Anything that needs a store builds a
+throwaway repository in a `tempfile.TemporaryDirectory()` — a `.git` marker, a
+copy of the script layers, fixture issues — and runs the real scripts inside it
+as subprocesses. That is the only way to test behavior that depends on where a
+script is run from, and it keeps the developer's own store out of the blast
+radius.
 
 ## Local issue store
 
@@ -75,6 +94,14 @@ the domain layer, it is in the wrong place.
 markdown file per issue, named by its slug, with one metadata field per line so
 plain grep works without a parser.
 
+- **The path is `<repo root>/tmp/issues`, resolved from `issue.py`'s own
+  location, not from cwd.** `issue.store_root()` walks up from `__file__` to the
+  nearest `.git` or `AGENTS.md` — so every script in both layers sees one store
+  whatever directory it is run from. An explicit `--out` overrides it and is
+  used exactly as typed; a relative `--out` stays relative to cwd.
+- Nothing creates the store as a side effect of a write. Readers distinguish
+  "does not exist" from "is empty"; only `issue_new.py` and `pull.py` create it,
+  and they say so on stderr.
 - Identity is the slug (`wire-sqlc-appclick.md`), never a tracker number.
   Numbers live in the `gitea:` field.
 - `origin: local` is a durable state. An issue that never leaves this machine is