v0.9.5: a session owns a directory, and the store hands out uuids

A session's transcript lived beside its attachments, both named after an id
that was a timestamp. Two things fell out of that: on a workspace two
machines share, a timestamp is not an id (the same second on both is enough
to hand out the same name), and "delete this conversation" had to chase the
jsonl, the title and a directory in a shared attachments/ tree, guessing what
belonged to whom. A phone's app data has nobody to tidy it by hand.

Now one session is one directory:

  <workspace>/.agent/sessions/
    <uuid>/session.jsonl     the transcript
    <uuid>/title             the window title, one line
    <uuid>/attachments/      images and delivered files (img-…, art-…)
    <uuid>/sub-1.jsonl       an agent role's own transcript
    CURRENT                  the id the next run opens

- src/agent_store.c: new_id() makes a uuid v4 (16 bytes from /dev/urandom,
  clock+pid only as a fallback), dir_for_id/flat_for_id/nested_for_id/
  shape_of_id decide which shape an id has, so every reader takes either.
  An id is a directory name, so it is checked as one (no "/", no "..",
  nothing starting with ".", not CURRENT or attachments). path_for_id() keeps
  the shape a session already has and makes the directory for a new id, so
  nothing had to change in the callers. store_list() lists the directories
  that hold a transcript plus the flat files, newest first. delete() is one
  rm_tree of the session's directory, plus the old shared attachments/<id>
  and CURRENT when it pointed there.
- cst_agent_store_migrate() (--migrate-store, or plain cst-agent for a
  session): moves a flat store in — <id>.jsonl to <id>/session.jsonl,
  <id>.title in, the shared attachments/<id>/ to <id>/attachments/. rename
  first, copy when the filesystem says no, and counts what it moved, so a
  second run is a no-op that says "nothing to move". On the real workspace:
  6 sessions moved, the shared directory gone, --list-sessions reads them
  back unchanged, a second run moved 0.
- cst_agent_store_id_of_path(): the id of a session path. This is the bug the
  whole change had been hiding: cst_agent_session_id() took the id from the
  file name, and the new file name is session.jsonl -- every session would
  have called itself "session" (the sidebar, CURRENT, undo, an agent role's
  directory, and the metrics file, which now also goes through the helper).
- src/attach.c: a session that owns its directory keeps its payloads inside
  it (<id>/attachments/); only the flat layout needs the shared directory and
  the id stem. Attachments behave exactly as before otherwise.
- tests/test_store.c: uuid shape and uniqueness, a small buffer refused, a
  new session's path and directory, write/list/info/title read back, the
  payload landing in <id>/attachments/, delete taking the directory with it;
  then an old-layout store: readable before the move, moved byte for byte,
  its payload readable through the new path, listed once, a second migration
  a no-op, deletion covering the migrated payload. Also in the Makefile and
  CMakeLists test lists.
- app/src/ui/UiTest.cpp: the image directory check wanted "/attachments/",
  which the new layout has no trailing slash for.

Desktop: build clean, make check 35/35, --uitest all green, --selftest and
cst-app-selftest OK, and --selftest opens a session of the migrated workspace.

Version 0.9.5.

Makefile

@@ -139,6 +139,7 @@ TESTS = \
 	tests/test_sse_decode \
 	tests/test_apply_patch \
 	tests/test_session_roundtrip \
+	tests/test_store \
 	tests/test_attach \
 	tests/test_artifacts \
 	tests/test_pdf_text \