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/README.md

@@ -141,14 +141,16 @@ skills/
       issue_ac.py                list the body's checkboxes; tick one
       issue_tree.py              draw the dependency graph
       issue_evict.py             drop closed issues the tracker also has
-      issue_index.py             rebuild tmp/issues/INDEX.md
+      issue_index.py             rebuild .tea/issues/INDEX.md
+      issue_init.py              create the .tea/ marker that makes a
+                                 directory a project; migrates tmp/issues in
   sync/                      /tea:sync — the bridge to Gitea
     SKILL.md
     scripts/
       map.py                     md <-> Gitea JSON, pure functions, no I/O
       _gitea.py                  transport: login pin, tea api, pagination, filters
-      pull.py                    Gitea -> tmp/issues/
-      push.py                    tmp/issues/ -> Gitea, then drops the local file
+      pull.py                    Gitea -> .tea/issues/
+      push.py                    .tea/issues/ -> Gitea, then drops the local file
       remote.py                  discovery listing to stdout
       comment.py                 post or edit a comment
       close.py                   the state field, both ways
@@ -164,9 +166,23 @@ ever disagree, `AGENTS.md` is the one being worked from.
 
 ## Local issue store
 
-Issues live in `tmp/issues/` (gitignore it) as flat markdown with one metadata
-field per line — so `grep -l 'labels:.*type/bug' tmp/issues/*.md` works without
-a parser.
+Issues live in `.tea/issues/` as flat markdown with one metadata field per line
+— so `grep -l 'labels:.*type/bug' .tea/issues/*.md` works without a parser.
+
+**Run `issue_init.py` once per project.** It creates the `.tea/` marker, which
+is what every script resolves the store from: they walk up from the working
+directory to the nearest one. The marker is never inferred from the tree, and
+with none anywhere the commands stop and name the directories they searched
+rather than picking a plausible one.
+
+```bash
+python3 <plugin>/skills/issue/scripts/issue_init.py
+```
+
+It is idempotent, adds `.tea/` to `.gitignore`, and moves an older
+`tmp/issues` store in if it finds one. Don't run it inside a git worktree: the
+marker is gitignored, so a worktree has none by design and reaches the main
+checkout's store on its own — exactly like the login pin.
 
 An `origin: local` file **is** the issue — the store, and the only copy.
 Anything with `origin: gitea` is a working copy of something the tracker