Gateway half of relay-platform-parity Phase 2.5 (D-Q2.5). The relay wire's
platform-neutral scope discriminator is renamed guild_id → scope_id; this is the
hermes-agent side of the cross-repo wire-compatible migration.
- SessionSource: scope_id is canonical; guild_id kept as @deprecated alias.
__post_init__ mirrors the two so all existing SessionSource(guild_id=...)
constructors across native adapters keep working unchanged. to_dict dual-WRITES
scope_id+guild_id; from_dict dual-READS scope_id ?? guild_id.
- relay/adapter.py: capture + outbound metadata dual-read/write scope_id.
- relay/ws_transport.py: _frame_to_event dual-reads scope_id ?? guild_id.
- docs/relay-connector-contract.md: document scope_id (canonical) + guild_id
(deprecated alias) in the §3 SessionSource field table (conformance test).
250 relay+session+contract tests green. Solo lane (relay).
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