feat(core): admit v2 skill guidance (#30843)

specs/v2/schema-changelog.md

@@ -768,3 +768,26 @@ Change:
 Compatibility:
 
 - Watcher-backed per-file `Refreshable` instruction observations, configured sources, nested discovery, and plugin-defined context remain follow-up slices.
+
+## 2026-06-05: Admit Selected-Agent Skill Guidance
+
+Affected schema:
+
+- Add `session_context_epoch.agent` so each durable baseline records its owning effective agent.
+- No synchronized event, public HTTP API, or generated SDK schema changes.
+
+Change:
+
+- Compose selected-agent, permission-filtered available-skill guidance with Location-wide System Context before Context Epoch admission.
+- Keep skill bodies behind the existing permission-checked `skill` tool and remove the unfiltered skill list from its Location-wide definition.
+- Stop missing-skill errors from enumerating the unfiltered Location-wide skill catalog.
+- Bind local tool authorization and pending permission requests to the provider turn's effective agent.
+- Keep absolute skill locations out of available-skill guidance; expose body and location only through the permission-checked `skill` tool.
+- Request Context Epoch replacement after an agent switch, dynamically re-observe the effective agent during retries, and fence first-epoch creation against the authoritative effective agent.
+- Fence existing-epoch replacement against the authoritative effective agent and block cross-agent provider turns while replacement context is unavailable.
+- Group the System Context algebra, registry, and built-ins under `system-context/`; keep source producers and Context Epoch persistence with their owning Skill, instruction, and Session modules; rename projected conversation selection to Session History.
+- Add the canonical V1-to-V2 runtime-context parity checklist to `specs/v2/session.md`.
+
+Compatibility:
+
+- Existing Context Epoch rows backfill the default `build` agent and reconcile to another selected agent at the next safe provider-turn boundary.