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.

docs/rfc/INDEX.md

@@ -123,6 +123,7 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
 | [Prompt variables and tool-guidance ownership](implemented/architecture/2026-07-05-prompt-variables-and-tool-guidance-ownership.md) | 2026-07-05 |
 | [Every LLM request is reconstructable from the session log](implemented/architecture/2026-07-05-reconstructable-requests.md) | 2026-07-05 |
 | [Subagent provider-lifecycle events — `subagent/provider-added` / `subagent/provider-removed`](implemented/architecture/2026-07-05-subagent-provider-lifecycle-events.md) | 2026-07-05 |
+| [The agent is a registration scope](implemented/architecture/2026-07-08-agent-scope-contexts.md) | 2026-07-08 |
 
 ### Process