Keep state and keys out of the shared repo

The dev VM is treated as untrusted, and the repo is a read-write virtiofs
mount shared with the backend host -- so a CA private key sitting in it is
a key that machine can sign with, and a leaf signed by this CA is one the
phone's pinned app accepts without question. Pinning against a CA the
attacker holds is no pinning at all.

So certificates are now generated on the machine that serves them, into
$XDG_CONFIG_HOME/ai-app/certs at 0700 with 0600 keys (AI_APP_CERTS
overrides), and config.json and session transcripts move to the XDG config
and data directories. Transcripts move for a plainer reason than the keys:
they are whole conversations, and they were world-readable at 0644.

Two smaller things fall out. The host and VM stop sharing one config,
which had already put a test token on the production backend. And state
stops living where `git clean -xdf` would take the enrollment and every
transcript with it.

State that predates the move is still read from the repo, with a warning
naming where to move it, so an existing install keeps working rather than
silently coming up on an empty config -- the precedence is covered by a
test, since picking the wrong file would otherwise be silent.

Verified: 31 tests, clippy clean; the certificate script writing 0700/0600
into an overridden directory; and the server logging the fallback and
serving from it.

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

server/src/session/claude.rs

@@ -629,7 +629,9 @@ impl Translator {
         let name = format!("{}.{extension}", super::random_hex());
         let dir = self.session_dir.join("files");
         if let Err(err) =
-            std::fs::create_dir_all(&dir).and_then(|_| std::fs::write(dir.join(&name), bytes))
+            crate::config::create_private_dir(&dir)
+                .map_err(std::io::Error::other)
+                .and_then(|()| std::fs::write(dir.join(&name), bytes))
         {
             tracing::error!("couldn't save produced image: {err}");
             return None;