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.

scripts/doc-budgets.manifest.json

@@ -1,7 +1,7 @@
 {
   "AGENTS.md": 1691,
   "docs/AGENTS.md": 1315,
-  "docs/architecture.md": 1640,
+  "docs/architecture.md": 1790,
   "docs/cordis-primer.md": 550,
   "docs/defensive-patterns.md": 550,
   "docs/testing.md": 800,