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_index.py

@@ -7,8 +7,12 @@ the index acknowledges that a tracker exists: `local` means the issue has never
 left this machine, `gitea` means the sync layer has pushed or pulled it. Both
 are ordinary issues here.
 
+The store is <repo root>/tmp/issues unless --out says otherwise; an existing
+store with nothing in it gets an "_empty_" table, a store that is not there is
+an error rather than a directory to create.
+
 Usage:
-    issue_index.py [--out tmp/issues]
+    issue_index.py [--out DIR]
 """
 import argparse
 import os
@@ -27,6 +31,9 @@ def cell(v):
 
 
 def build(root):
+    # An index of a store that is not there is not an empty index, it is a bad
+    # path. Raising beats writing INDEX.md into a directory nobody asked for.
+    issue.require_store(root)
     issues = issue.load_all(root)
     rows = []
     for i in sorted(issues):
@@ -71,7 +78,6 @@ def build(root):
 
     out.append("")
     path = os.path.join(root, "INDEX.md")
-    os.makedirs(root, exist_ok=True)
     with open(path, "w") as f:
         f.write("\n".join(out))
     return path, len(rows)
@@ -79,9 +85,16 @@ def build(root):
 
 def main():
     ap = argparse.ArgumentParser(description="Rebuild the local issue index (offline)")
-    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()
-    path, n = build(args.out)
+    # An existing store with nothing in it is a legitimate thing to index — it
+    # gets an "_empty_" table. A store that is not there is not.
+    try:
+        path, n = build(args.out)
+    except issue.StoreMissing as e:
+        sys.exit("issue_index.py: %s — nothing was created; create an issue with "
+                 "issue_new.py, or pass --out" % e)
     print("%s — %d issue(s)" % (path, n))