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/references/format.md

@@ -10,7 +10,7 @@ sync layer's business — see `/tea:sync`.
 
 ## Identity
 
-An issue is one file, `tmp/issues/<id>.md`, and `id` is a slug: lowercase
+An issue is one file, `.tea/issues/<id>.md`, and `id` is a slug: lowercase
 ASCII, digits, single dashes, derived from the title. **The slug is the
 identity.** It is stable for the life of the issue — a retitled issue keeps its
 slug; an issue pushed to a tracker, deleted locally and fetched back a month
@@ -18,7 +18,7 @@ later keeps it too. Tracker numbers are a foreign key stored in a field, never
 the name of anything.
 
 ```
-tmp/issues/wire-sqlc-appclick.md
+.tea/issues/wire-sqlc-appclick.md
 ```
 
 A slug never contains a dot, which is how the store tells an issue from the
@@ -86,7 +86,7 @@ It is not a *permanent* state, and it is what the file's fate depends on:
 | `local` | the issue itself — the only copy there is | creates it in the tracker, then deletes the file | **nothing, ever** — in any state, named or not |
 | a tracker | a working copy of something the tracker already has | updates the tracker, then deletes the file | removes it once `state: closed` |
 
-**A successful push deletes `tmp/issues/<id>.md`** (and `<id>.comments.md`), on
+**A successful push deletes `.tea/issues/<id>.md`** (and `<id>.comments.md`), on
 create and on `--update` alike. What is in the store is what has not left this
 machine; everything else is fetched again when it is needed. The rule, its
 safety conditions, and how the slug survives are `/tea:sync`'s to state.
@@ -183,7 +183,7 @@ sections the reference actually came from.
 Draw the graph with `issue_tree.py`. The reverse direction is a grep:
 
 ```bash
-grep -ln 'depends:.*migrate-schema' tmp/issues/*.md
+grep -ln 'depends:.*migrate-schema' .tea/issues/*.md
 ```
 
 ## Shared rules