runtime/session: codec reconcile, managed-tools algebra, tool-surface refresh, rebindable stores, usage rollup #476

Generic session-runtime pieces OpenHuman carries in agent/session_host/ and agent/session_store/ (see openhuman#7929):

• Default TranscriptCodec::reconcile: match next messages to unused prior rows. Hosts keep only metadata/usage callbacks. From session_host/codec.rs (353).
• managed_tools set algebra: visible / deferred / permanent (runtime_session/permanent.rs).
• Per-turn synthesized tool-surface refresh and visibility diff in ToolSetBuilder / tool_registration (runtime_session_tool_surface.rs, builder/mod.rs name dedup).
• TurnControls parity: pause_at_cap and defer_turn_completed, so session_host/turn/graph.rs can be deleted.
• CommitReceipt history rows and AgentError kind mapping (runtime/turn_helpers.rs).
• Rebindable DriverSessionStores: swap in place with a live-handle registry (session_store/rebound.rs). Rename away from StoreRegistry, which collides with the harness store::StoreRegistry.
• Subagent usage/cost roll-up in harness cost (runtime_session_usage.rs).

Done when: merged upstream with tests in sibling *_tests.rs, released or tagged, and the OpenHuman follow-up can delete the host copy after a gitlink bump on vendor/tinyagents.

Parent: tinyhumansai/openhuman#7929, which has the full file-by-file verdict tables.