Drop the in-repo state fallback

Nothing has run against the host's backend yet, so there is no old config
or transcript to keep working -- the XDG paths are simply where state
lives. Removing the fallback takes repo_root() with it, since finding the
repo from the running executable existed only to locate that legacy state.

AGENTS.md gets the arrangement that replaced it: the host builds and runs
from its own clone outside the shared mount, and code reaches it by push
to gitea, which this VM's key is not authorized for.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xn8nHw1tw1R6PtiY1eEtw

PLAN.md

@@ -255,9 +255,7 @@ GET/PUT /hosts, /models               config editing from the phone
 Sessions live in `config.json` (`$XDG_CONFIG_HOME/ai-app/`) + a per-session
 directory under `$XDG_DATA_HOME/ai-app/sessions/` (transcript.jsonl,
 attachments, produced images), owner-only. Deleting a session is the
-complete path out of everything spawning one created. State that predates
-the move is still read from the repo, loudly, so an existing install keeps
-working until it is moved by hand.
+complete path out of everything spawning one created.
 
 ### Security