docs(architecture): add OQ-19 session-scoped registries and agent-written operations

Document the three-tier registry model (core/session/promotion) and the
self-improving agent workflow where agents write their own operations in
a quickjs sandbox. The POC at /workspace/toolEnv demonstrated the sandbox
mechanism (quickjs in Deno web workers, proxy-based env bridge via
postMessage) but exposed the full registry to the sandbox — the security
gap that OQ-18's scoped composition env addresses.

The call protocol doesn't need changes: the OperationEnv trait is the
composition point, and a session-scoped env wraps the global env (session
registry first, fall through to global). The one-way door this OQ guards
against: making OperationEnv concrete instead of a trait, or hardcoding
the global registry into the dispatch path, would close the session-overlay
pattern. Session-scoped operations are always Internal, run under the
handler's identity, and are ephemeral. Promotion to core requires curation
review (architect role with promote scope).

docs/architecture/crates/call/README.md

@@ -41,6 +41,7 @@ Structured RPC over QUIC: operations, request/response, streaming subscriptions,
 | OQ-16 | Safe vault operations for call protocol exposure | resolved (ADR-014) | None exposed for now |
 | OQ-17 | Abort cascade semantics | open | `call.aborted` cascades to descendants; default `abort-dependents`, `continue-running` opt-in. One-way door on event schema |
 | OQ-18 | Privilege model and authority context | open | `internal` flag switches authority to handler identity, not blanket ACL skip. External/Internal operation visibility. Scoped composition env + handler identity. Protocol-level concern |
+| OQ-19 | Session-scoped operation registries | open | Agent-written operations overlaid on global registry via `OperationEnv` trait layering. Protocol doesn't need changes; one-way door is not closing the trait-based composition point |
 
 ## Key Design Principles