Merge origin/master into worktree/web-plugin-config

Resolves the vendored-package rescope (cordis -> @deepseek-ai/cordis,
schemastery -> @deepseek-ai/schemastery) against this branch's settings
namespaces: the bash capability namespace, its two executors' section
installs, and the new plugin-config client package all move to the scoped
specifiers.

docs/subsystems/core.md

@@ -419,6 +419,45 @@ async resolve(id?: string): Promise<AgentPreset>
  */
 async mount(agentCtx: Context, id?: string): Promise<AgentPreset>
 
+/**
+ * Join one agent to the SAME standing composition another already runs on.
+ *
+ * This is how a child agent inherits its parent's capabilities. It is a bind,
+ * not a mount: the parent's generation is already composed, so the child gets
+ * that exact instance — the same plugin objects, the same tool registrations,
+ * the same prompt sections. Re-resolving the parent's preset by id instead
+ * would re-read the roster, and a composition file edited since the parent
+ * started would hand the child a DIFFERENT generation than the one its
+ * parent's history was produced under (and a preset deleted since would fail
+ * the child outright while its parent keeps running).
+ *
+ * Synchronous, and with no composition failure mode of its own — it reads no
+ * roster, mounts nothing, and touches no file — which is what lets a child
+ * creation window use it: the two in-process subagent drivers compose their
+ * children inside a synchronous `setup`. It still rejects a caller error, as
+ * the `@throws` below record.
+ *
+ * A parent that joined no preset — a rosterless deployment — yields no join
+ * and no error: there, the model-facing rows sit in the host composition and
+ * the child already sees them through the global layer.
+ * @param agentCtx - the joining agent's scope context.
+ * @param parentCtx - the scope context of the agent whose composition to join.
+ * @returns the preset id joined, or undefined when the parent joined none.
+ * @throws when `agentCtx` carries no scope, or has already joined a preset.
+ */
+composeFrom(agentCtx: Context, parentCtx: Context): string | undefined
+
+/**
+ * The preset one live agent runs on.
+ *
+ * Read from the live scope chain rather than from the session, so it answers
+ * for an agent whose session has not recorded a preset yet — a child agent
+ * whose durable header is being built from its parent's composition.
+ * @param agentCtx - the agent's scope context.
+ * @returns the preset id, or undefined when the agent joined none.
+ */
+composedPreset(agentCtx: Context): string | undefined
+
 /**
  * Read one preset's composition text.
  * @param id - the preset id.