Delete stored compatibility encodings

[SaladDay]

Summary

Three pre-release compatibility paths are deleted. None of them has a producer any more. This is part of the AGENTS.md alignment (T3c in the architecture audit).

• {"text": …} stored input.
  • Production stores the public agent.session.input.message event (api/inputs.go).
  • The only writer of the legacy shape was a test fixture, which now submits the public event.
  • items/inputs.go and execution/message_input.go drop their Text branches.
• Item.MarshalStored.
  • Stored Item payloads now use the wire encoding (MarshalJSON), so there is one Item encoding.
  • Replayed child Items still compare equal, because both sides use the same encoder (payload = candidate::jsonb).
• Sessions stored without a model provider.
  • Creation already rejects these Sessions, and every row carries the provider flag.
  • The three execution guards are deleted, with ErrModelProviderRequired, its worker outcome and failure branches, and its 400 mapping.
  • The sessions-events.md sentence is removed (en and zh).

Behaviour

• Child Item rows written before this change use the old encoding and compare unequal on replay. There is no migration (pre-release, no compatibility).
• A stored {"text"} input no longer projects or dispatches. Nothing writes it.
• The stored model_provider_required failure code stays readable for existing rows. Only its producers are gone.

Checks

• go build ./..., module vet and gofmt.
• DB tests:
  • items 28, sessions 287, sessionpg 98, v1 53 and execution 220;
  • api 120, with -run limited to the diagnostics, session and error-mapping tests;
  • tests/integration 738 passed, plus the fixed tests and the official-client tests rerun with the pinned SDK.
• official_client.py, check-names.py, and the website translation and build tests.

Net: non-test −57 lines, test −97 lines.

🤖 Generated with Claude Code

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