feat: drop the local copy after a successful push

Gitea becomes the source of truth. Once a push is confirmed, push.py
deletes tmp/issues/<id>.md and <id>.comments.md and prints the number and
URL the issue now lives at; the current state is obtained by pulling
again rather than by reconciling. --update follows the same rule, with no
exception: what is local is what has not left.

This reverses three statements AGENTS.md used to make, and rewriting them
is part of the change:

  - "tmp/issues/ is the store, not a cache of Gitea" — it is both, split
    by origin:. An origin: local file is the only copy of the work; an
    origin: gitea file is a deletable working copy.
  - "Pushing is additive: the file is never deleted" — it is deleted.
  - "origin: local is a durable state" — complete, but not durable:
    pushing ends it.

Slug stability, which the format promises for the life of an issue, can
no longer rest on a file push is about to delete. The slug goes up in the
body as a hidden marker, <!-- tea:id <slug> -->, on the first line:
map.to_payload strips every marker and prepends exactly one, map.from_api
strips every marker on the way down, so the local file never holds one
and a body cannot accumulate them however many round trips it makes. The
marker survives a rename in the web UI, a lost .remote.json, a fresh
clone and another machine — none of which a local index does.

Deletion is the last thing that happens to an issue and only after the
transport returned, the answer carried a positive integer number (and, on
--update, the number that was PATCHed — push.confirmed_number), and
.remote.json was written. A raised transport, a non-2xx, an empty or
mismatched body each leave the file on disk and stop the run.

.remote.json is no longer "only an index over the files": its entries now
deliberately outlive them, so it is the local number -> slug ledger and
rebuild_map merges into it instead of reconstructing it from files that
may be gone. It stays recoverable, from the markers in Gitea rather than
from the files. push.dep_state reads it too, so a blocker whose file an
earlier push dropped still gets its native dependency link.

Also fixes a pre-existing bug the new tests hit: issue.all_ids treated
<id>.comments.md as an issue called "<id>.comments", so a bare push.py in
a store holding pulled threads tried to file a comment thread as a unit
of work. A slug has no dot in it.

tests/test_drop_after_push.py covers the round trip (push -> gone -> pull
-> identical in slug, depends: and body), the marker's algebra, and every
failure path separately. test_push_dependencies.py is updated where it
encoded the old "never deleted" contract. 183 tests, no network.

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

skills/issue/SKILL.md

@@ -115,8 +115,11 @@ Edit the file. Change `state:` to close it, edit `labels:`, add ids to
 `depends:`. Re-run `issue_check.py` afterwards, and `issue_index.py` to refresh
 the table. Checkboxes are the exception — use `issue_ac.py`, below.
 
-If the issue is synced (`origin: gitea`), your edit is local until you run
-`push.py --update` from `/tea:sync`. Nothing tracks that drift automatically.
+If the issue is synced (`origin: gitea`), the file is a working copy: your edit
+is local until you run `push.py --update` from `/tea:sync`, and that push
+**deletes the file** once Gitea has it. Nothing tracks drift, and with one copy
+at a time there is little to track — a file that is still here has not been
+pushed. Get it back with `pull.py <n>`; the slug does not change.
 
 ## Ticking checkboxes