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>

skills/issue/SKILL.md

@@ -47,6 +47,24 @@ tmp/issues/wire-sqlc.comments.md    comment thread (written by /tea:sync only)
 tmp/issues/tree-<id>.md             saved graph (issue_tree.py --write)
 ```
 
+## Where the store is
+
+`<repo root>/tmp/issues` — **not** `tmp/issues` relative to wherever you are
+standing. The scripts resolve it by walking up from their own file to the
+nearest `.git` or `AGENTS.md`, so they all see one store no matter which
+directory you run them from, and a `cd` earlier in the session changes nothing.
+
+`--out` overrides that and is taken **literally**: an absolute path is used as
+given, a relative one stays relative to the current directory. Nothing rewrites
+what you typed.
+
+Two things follow, and both are deliberate:
+
+- A store that is not there reports `does not exist`; a store with no issues in
+  it reports `is empty`. They are different problems.
+- No script conjures a store as a side effect of writing. Only `issue_new.py`
+  creates one — the first issue in a fresh checkout — and it says so on stderr.
+
 ## Reading: grep, don't parse
 
 Metadata is one field per line with inline lists precisely so plain `grep`