refactor(relay): purge platform-specific scope terminology from the relay adapter (D-Q2.5c) (#56016)

The gateway HALF of the D-Q2.5c cleanup (connector half: gateway-gateway #92).
Scope is STRICTLY the relay adapter (gateway/relay/) — session.py and every
native platform adapter are untouched (SessionSource.guild_id remains for their
use; it is NOT relay-only).

Within gateway/relay/, drop the D-Q2.5 wire dual-write/dual-read alias AND
genericize all platform-specific (Discord "guild") scope terminology:
- ws_transport._event_from_wire: read scope_id only (drop the ?? guild_id fallback).
- adapter._with_scope: emit scope_id only on outbound metadata (drop the
  guild_id dual-write); genericize the "GUILD reply" docstring to "SCOPED reply".
- adapter._capture_scope: read source.scope_id only; rename the local `guild`
  var to `scope`; genericize the docstring + the _scope_by_chat/_dm_user_by_chat
  field comments ("guild_id (Discord)" -> "scope_id (server/workspace scope)").
- __init__.relay_route_keys docstring: "guild_ids" -> "scope_ids".
- The ONE real Discord `guild_id` kept: the raw inbound interaction payload
  field (payload.get("guild_id")), which is Discord's own wire field, mapped
  straight into the generic scope_id slot — unchanged.

Contract doc (docs/relay-connector-contract.md): reframe the `guild_id` row as
a legacy alias the connector no longer reads (session.py's agent-wide to_dict()
still emits it for non-relay persistence, so it stays documented + wire-present
but ignored) — accurate, and keeps the to_dict()-vs-doc conformance test green.

Tests (relay only): migrate the wire-key writes + assertions guild_id -> scope_id
across test_relay_adapter / _ws_transport / _passthrough / _roundtrip /
_roundtrip_telegram / _multiplatform; keep raw Discord `type:2` interaction
payloads' guild_id (real Discord field) and the conformance test's guild_id
parametrize (validates the kept legacy field stays wire-reachable).

Gate: 156 relay tests pass, ruff clean. Cross-repo E2E — all 14 drivers pass
BOTH ways: connector#92 (scope_id-only) x agent-main (still dual-reads) AND
connector#92 x this worktree (scope_id-only). Deploy-order-safe either way.

docs/relay-connector-contract.md

@@ -157,7 +157,7 @@ present (may be `null`); the rest are included only when set.
 | `user_id_alt` | string | no | Platform-specific stable alt id (Signal UUID, Feishu union_id). |
 | `chat_id_alt` | string | no | Alternate chat id (e.g. Signal group internal id). |
 | `scope_id` | string | no | Platform-neutral **scope** discriminator: Discord guild / Slack workspace / Matrix server. **REQUIRED for Discord/Slack scope isolation.** Session-key discriminator. (Canonical name as of the D-Q2.5 wire migration.) |
-| `guild_id` | string | no | **Deprecated alias for `scope_id`** — still emitted and read during the cross-repo dual-read/dual-write overlap; readers resolve `scope_id ?? guild_id`. Dropped once both repos deploy on `scope_id`. |
+| `guild_id` | string | no | **Legacy alias, no longer read by the connector.** As of D-Q2.5c the connector reads and writes only `scope_id`; the gateway's agent-wide `SessionSource.to_dict()` still emits `guild_id` (mirrored to `scope_id`) for non-relay session persistence, so it may still appear on the wire but the connector ignores it. Do not depend on it. |
 | `parent_chat_id` | string | no | Parent channel when `chat_id` refers to a thread. |
 | `message_id` | string | no | Id of the triggering message (for pin/reply/react). |
 
@@ -168,7 +168,7 @@ present (may be `null`); the rest are included only when set.
 
 ### SessionSource discriminators per platform
 
-| Platform | chat_id | chat_type | user_id | thread_id | guild_id |
+| Platform | chat_id | chat_type | user_id | thread_id | scope_id |
 | --- | --- | --- | --- | --- | --- |
 | **Discord** | channel id | `dm`/`group`/`thread` | author id | thread channel id (threads) | **guild id** (REQUIRED for server isolation) |
 | **Telegram** | chat id | `dm`/`group`/`forum` | from id | forum topic id (forums) | — |