Foundations for serving multiple profiles from one gateway process, inert
when off:
- gateway.multiplex_profiles config flag (default false), round-trips through
GatewayConfig and load_gateway_config (top-level + nested gateway.* form).
- hermes_cli.profiles.profiles_to_serve(multiplex): the single chokepoint for
which (profile, HERMES_HOME) pairs the gateway serves. Lightweight dir scan;
active-profile-only when off, default + all named profiles when on.
- build_session_key gains a profile= namespace slot. Default/None reuse the
historical 'agent:main:...' literal BYTE-IDENTICALLY (no session migration,
positional parsers unaffected); a named profile becomes 'agent:<profile>:...'
so two profiles on the same platform/chat never collide.
- SessionStore._resolve_profile_for_key + _session_key_for_source fallback
resolve the namespace from the flag (legacy when off, active profile when on).
Tests: byte-identical-when-off (parametrized), namespace isolation, positional
layout preserved, config round-trip, profiles_to_serve enumeration.
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