fix: resolve the issue store from the project, not the plugin

`issue.store_root` and `_gitea.PAYLOAD_ROOT` were anchored on `__file__`, on
the reasoning that where an installation keeps its files is a fact about the
installation. That holds for an installation and not for a store.

Installed, the plugin therefore resolved every project's issues inside its own
directory — and a plugin cache is versioned, so the store moved on each
update:

    ~/.claude/plugins/cache/tea/tea/2.0.0/tmp/issues   5 files, 2 origin: local
    ~/.claude/plugins/cache/tea/tea/2.1.0/tmp/issues   12 files
    ~/.claude/plugins/cache/claude-skills/tea/2.2.0/   empty, the current one

Issues written from one project were invisible from the next, and an `origin:
local` file — which IS the issue, the only copy — was stranded a version bump
at a time. Two of them were.

The store is a fact about the project, exactly as the login pin is. So the
anchor is now an explicit marker an operator creates, `.tea/`, searched for up
from $CLAUDE_PROJECT_DIR and then cwd — the pin's order, so the two cannot
disagree about which project this is. Inferred markers were tried and are worse
than useless here: `.git` is in every clone including this plugin's own, and
the agents-sync hook writes an AGENTS.md next to every AGENTS.md, so the plugin
root always carried one and cwd never got a turn.

With no marker anywhere, `store_root()` is None and every entry point reports
which directories it searched. A store in a plausible-looking directory is the
failure this replaces, so nothing falls back to one.

- `.tea/` holds the store and the transport's scratchpad: `.tea/issues`,
  `.tea/payload`. One marker, one walk, one gitignore line.
- `issue_init.py` creates it, moves an old `tmp/issues` store in rather than
  copying, adds `.tea/` to `.gitignore`, and refuses to pick a winner when both
  sides hold the same file name.
- A linked worktree has no marker — it is gitignored — and reaches the main
  checkout's store by the hop the pin already took.
- `parents`, `gitdir_of` and `main_worktree` move from `pin.py` into the domain
  and `pin.py` imports them. The domain depends on nothing, so it is the layer
  all three callers can borrow from, and the walk stays written once: the
  guard, the transport and the store cannot disagree about a directory.

The suite stopped copying the script layers into its fixtures. That is what hid
this: with the scripts inside the fixture, the installation and the project
were the same directory. They are now deliberately far apart, and a regression
test asserts the plugin tree gains no files when commands run against a project
somewhere else.

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

plugins/tea/skills/issue/scripts/issue_index.py

@@ -1,13 +1,13 @@
 #!/usr/bin/env python3
 """
-issue_index.py — rebuild tmp/issues/INDEX.md from what is on disk. Offline.
+issue_index.py — rebuild .tea/issues/INDEX.md from what is on disk. Offline.
 
 A map of the local store, nothing else. The `origin` column is the only place
 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
+The store is <project root>/.tea/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.
 
@@ -100,7 +100,7 @@ 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: <repo>/tmp/issues)")
+                    help="store root (default: <project>/.tea/issues)")
     args = ap.parse_args()
     # 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.