backend-agents H24: docs/session-model.md + docs/agent-stream.md durable/attach wire

[btipling]

Parent: #794
Umbrella: #794
Label: backend-agents
Status: DRAFT
Blocked on: #817 (H23)

Implements one row of the restart umbrella. One issue → one PR → one adversarial review → one merge → close. Do not expand scope.

Slice

Reserved-meta table for A1–A3 + cursor + checkpoint pointer. Durable events. Re-resolve every tool and model step. Checkpoint-as-Blob.

Timeless; no issue archaeology.

DoD

• docs/session-model.md + docs/agent-stream.md match shipped behavior
• Docs-only

Refs

• Parent/umbrella: plan: backend-agents RESTART — client-independent turns (umbrella, ≥20 small issues + handoff) #794