docs: agent-scope RFC, CONTEXT.md glossary, architecture scope section, README sync

The agent-scope-contexts RFC (implemented) records the decision tree:
the dsh-scope primitive over cordis extend/Context.filter/no-op fibers,
two-level flat scope with shadowing, restriction/grant semantics, the
scoped-dispatch rule with fused helpers, the setup window, and the
alternatives (explicit scope params, isolate, event-filtering-only,
vendored support) with why each lost. CONTEXT.md pins the glossary.
architecture.md gains the Agent Scope section, the dsh-scope spine row,
the scoped turn-flow line, and an extension-table row (ceiling 1640→1790:
the two-layer registration model is a new architectural axis; additions
are condensed to pointers). READMEs of every touched package re-state
their scoped facts; the stale structured-runtime README section is
replaced by the scoped-registration description.

packages/subagent/subagent/README.md

@@ -25,7 +25,7 @@ Unlike the bash seam (one executor per context, second load throws), **multiple
 
 ## Capabilities: two kinds, discovered two ways
 
-- **Start-time features** (`outputSchema`, `depthLimit`, `toolFilter`) are a static `provider.capabilities` descriptor, checked by the service BEFORE a run exists. A request that needs one the provider lacks is **rejected loud** (`UNSUPPORTED_CAPABILITY`), never accepted-then-ignored.
+- **Start-time features** (`outputSchema`, `depthLimit`, `toolFilter/persona`) are a static `provider.capabilities` descriptor, checked by the service BEFORE a run exists. A request that needs one the provider lacks is **rejected loud** (`UNSUPPORTED_CAPABILITY`), never accepted-then-ignored.
 - **Runtime features** (steering, resume) are **optional methods** on `SubagentRun` (`sendMessage?`, `resume?`). The method's presence IS the capability; TS narrowing is the discovery mechanism — a consumer cannot call an absent method without narrowing first, so there is no silent degradation path.
 
 Beside `capabilities` sits one DESCRIPTIVE fact, not validated by the service: `provider.inheritsParentContext` — whether a child sees the parent conversation (`fork`: true — seeded with the completed-turn prefix; `spawn`/`acp`: false). The model-facing consumer (`dsh-tool-subagent`) derives truthful tool wording from it.