lembas -l / --local: everything in the project, nothing written elsewhere

Config, data and state are rooted at <project>/.agent/local/lembas (or
.lembas/), kept out of git; a local directory the repository carries is
refused. The user's logins and connections.yaml are read, never written; the
user's config.yaml is not read. The TUI is started with TMPDIR inside it (the
compiled binary unpacks its native library there) and OpenTUI's cache goes
there too; the agent's commands keep the user's TMPDIR.

An instance serves models, chat, speech and the balance only: the link,
/remote, the service and the hub are off, deletes are not sent, and the server
library, search, fetch, embeddings and personalization are not used.
Self-update and uninstall are refused. hardline_disable is ignored, and tools
treat the local directory as outside the project.

The smoke test runs a whole local session in the compiled binary and checks
that HOME, the XDG directories and TMPDIR gained nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/git/snapshot.ts

@@ -1,6 +1,7 @@
 // Snapshots of the working tree in a shadow git repository — the idea of OpenCode's
-// snapshot/index.ts, written small. The shadow repo lives in ~/.local/share/lembas/snapshot/,
-// never inside the project, and works whether or not the project itself uses git.
+// snapshot/index.ts, written small. The shadow repo lives in ~/.local/share/lembas/snapshot/ —
+// in local mode (-l) in the project's git-ignored local directory, which it leaves out of every
+// snapshot — and works whether or not the project itself uses git.
 //
 //   track()            → a tree hash for the working tree as it is now
 //   changed(a, b)      → paths that differ between two trees