Settle the generated client review findings

[SaladDay]

Fixes from the batched blind review of #549 and #554.

• Generator. core_module now raises when a package-prefixed name or a hoisted inline enum would replace another type. Before, a second schema silently overwrote the first, and --check and tsc still passed. A test covers both cases.
• Subagent events. agent.session.subagent.* events no longer carry a top-level session_id. The pinned schema allows only type, event_id and subagent, and the Subagent already names the Session.
• Core errors. CoreAPIError.code and .param are always sent (nullable), so they are now tagged binding:"required".
• Docs. The coverage ledger (English and zh) states the swag tag convention for /core/v1 responses.

Checks: generator tests (16), the v1 and api tests, make check-openapi, client tsc and the translation check.

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