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/scripts/issue_new.py

@@ -151,7 +151,8 @@ def main():
     ap.add_argument("--assignee", action="append", default=[], help="assignee; repeat")
     ap.add_argument("--depends", action="append", default=[],
                     help="id this issue depends on; repeat")
-    ap.add_argument("--out", default=issue.ISSUE_ROOT, help="store root (default: tmp/issues)")
+    ap.add_argument("--out", default=issue.ISSUE_ROOT,
+                    help="store root (default: <repo>/tmp/issues)")
     args = ap.parse_args()
 
     labels = ["type/%s" % args.type]
@@ -177,6 +178,11 @@ def main():
         labels=labels, assignees=args.assignee, milestone=args.milestone,
         depends=args.depends)
 
+    # The first issue in a fresh checkout has to create the store, but it says
+    # so — and it says where, because the path is absolute.
+    if issue.create_store(args.out):
+        sys.stderr.write("created store %s\n" % os.path.abspath(args.out))
+
     path = issue.save(args.out, iss)
     issue_index.build(args.out)
     print("%s  [type/%s] %s" % (path, args.type, args.title))