`main` is a valid profile name (only hermes/default/test/tmp/root/sudo are
reserved), but _session_key_namespace mapped it to `agent:main` — the default
profile's namespace. Both profiles then built byte-identical keys: one routing
entry, one cached agent, and, since 75ae2859b9 pinned default-namespace
keys to the launch store, profiles/main's scoped sessions were written into
the ROOT state.db instead of profiles/main/state.db.
Key the `main` profile as `agent:main~` (`~` is outside the profile-id
alphabet, so the marked form cannot be any other profile's id) and give the
namespace slot one inverse, profile_from_session_key_namespace, used by the
store's key parser, _parse_session_key, the update-marker profile reader and
the profile-delete eviction prefix. Default keys stay byte-identical.
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