fix(agent): make Controller own canonical submit admission

[rv4no]

Summary

• Make Controller the sole owner of canonical user-message admission.
• Add durable submit intent and provenance updates keyed by clientSubmitId, with provider/session/turn delivery facts kept separate from canonical content.
• Separate composer intent, attachment preparation, and presentation state; add revision-based draft clearing and stale upload guards.
• Preserve prompt.id as queue identity distinct from clientSubmitId.

Why

The Controller, Provider runtime, and Host could all replay the same canonical user message. This makes admission durable before provider dispatch and makes retries idempotent state updates instead of payload rewrites.

Tests

• git commit pre-commit checks passed.
• pnpm check:changed -- --push-ready passed all 25 lanes.
• Targeted Agent GUI and Go tests passed during iteration.

Notes

• This is the upstream contract change consumed by the companion TSH PR.
• The branch intentionally remains behind main; no rebase was performed.

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