Stamp session metadata into `session.config`

[samueljklee]

Problem

app-cli stamps application_host, root_session_id, bundle_name, project_slug, project_name, working_dir, project_dir into session.config (the coordinator's config dict) at session_runner.py:159-223. Distro only writes some of these to metadata.json on disk — but never writes to session.config.

Hooks and modules reading session.config["application_host"] or session.config["root_session_id"] for tracing/observability get KeyError or None.

Evidence

• grep -rn "application_host\|root_session_id\|project_slug" distro-server/src/ returns zero matches in session_backend.py
• Distro writes session_id, bundle, working_dir, description to metadata.json via MetadataSaveHook — but not to session.config

Fix

After prepared.create_session() returns in both create_session() and _reconnect(), stamp:

session.config["application_host"] = "Amplifier Distro"
session.config["root_session_id"] = session_id
session.config["bundle_name"] = bundle_name or self._bundle_name
session.config["working_dir"] = str(wd)
session.config["project_dir"] = str(wd)
session.config["project_name"] = wd.name
session.config["project_slug"] = project_id

Use guard pattern if "key" not in session.config to prevent child sessions from overwriting inherited values.

~15 lines in session_backend.py, added in two places (create + reconnect).