Expose the lineage tree (siblings, children, live roots) to routes, not only the request's own chain #457

Problem

Since #421/#444, (await agent()).lineage answers "who is my parent, what is my root, am I a subagent" for the current request. It does not answer the other half of "where am I in the agent topology": who else is alive under my root (siblings), what did I spawn (children), and which other root conversations are live in this workspace.

A plugin that coordinates agent threads (the examples/worktree-proximity pattern: one root, several subagents in linked worktrees, warn/notify the other actor) therefore still has to hand-roll a topology state definition and feed it from session/start + agent/start + stop itself, even though the warm runtime already keeps exactly that tree in the agent lineage registry. The example does not use request.lineage at all (zero references under examples/worktree-proximity/src), because the axis cannot tell it who its peers are.

Evidence (main @ 284141958)

• packages/rsc-runtime/src/agent-request.ts:113-123 — AgentLineage is { conversation, depth, generation?, parent?, resolution, root, subagent? }: the request's own chain only.
• packages/rsc-runtime/src/lineage/state.ts:10-32, 49-66 — the registry already journals every node (id, parent, root, depth, startedAt, stoppedAt, type, isParallelWorker) in LineageState.nodes.
• packages/rsc-runtime/src/lineage/registry.ts:48-60 — AgentLineageRegistry.snapshot() is documented as "for dumps and the Workbench"; packages/agent-bundle/src/build/entry-shell.ts:1028-1041 hands the registry only to createGeneratedRouteMcpServer. No route surface (agent(), providers, services) can reach it.
• examples/worktree-proximity/src/state.ts:93-165 — the hand-rolled topologyStateDefinition (actors with kind: root|child, parentSessionId, status), fed by src/events/session/start.tsx, src/events/agent/start.tsx, src/events/stop.tsx.
• examples/worktree-proximity/README.md:47-49: "one workspace-durable topology definition and the framework notice definition share the generated runtime's SQLite driver"; README.md:70-81 documents the per-app actor bookkeeping this forces (native vs derived provenance, worktree:<root> fallback identity).
• Add typed Agent request context and pluggable context providers #95 listed "a built-in thread-topology algorithm" as a non-goal; feat(lineage): host-test probe, live host lineage matrix, and runtime-held request.lineage #421 then built the registry anyway to serve request.lineage. This issue asks only to read what that registry already holds — no new inference.

Proposed shape (small)

Extend the axis rather than adding a new one, so unavailability semantics stay identical:

interface AgentLineage {
  // …existing fields…
  /** Live nodes the registry currently holds under the same root, excluding this conversation. */
  readonly siblings?: readonly AgentLineagePeer[];
  /** Live nodes whose parent is this conversation. */
  readonly children?: readonly AgentLineagePeer[];
  /** Other live depth-0 conversations the registry holds (same workspace digest on Cursor). */
  readonly roots?: readonly AgentLineagePeer[];
}
interface AgentLineagePeer {
  readonly conversation: string;
  readonly depth: number;
  readonly parent?: string;
  readonly startedAt: string;
  readonly type?: string;
}

Fed from LineageState.nodes at resolve time (registry.ts resolve() / resolveToolCall()), pruned to stoppedAt === undefined. Alternative: a separate (await agent()).lineageTree() read so the per-request axis stays small; either is fine, the point is that routes can read the tree the runtime already keeps.

Design question: should roots be exposed at all (it crosses conversations that share nothing but an install), or only siblings/children under the caller's own root?

Out of scope / related

• Host-side gaps in building the tree stay on Claude Code 2.1.257 lineage gaps: SubagentStart names no parent; a subagent's hooks name no parent #422 / Codex 0.147.0 lineage gaps: SubagentStart names no parent thread; hooks lag the MCP _meta lineage #423 / Cursor 3.18.25 lineage gaps: subagentStart omits the child conversation id; child hooks carry no parent/root; MCP _meta carries no conversation; sessionStart not dispatched to plugin hooks on desktop #424.
• Addressing a notice at one of these peers is a separate issue (see "Related" comment below); this one is read-only.
• No operator/user identity (Actor principal for generated event scopes: typed unavailable reasons and the one derivable host field (parked from #233/#223) #391 decision, feat(lineage): request.lineage is the only identity-adjacent surface; scoped, self-correcting Cursor child binding #444).

Acceptance

In examples/worktree-proximity: delete src/state.ts's actor bookkeeping (keep activities/intent), and have src/events/tool/before.tsx find the other actor's conversation through lineage.siblings instead of topology.actors; the existing route-unit journeys 1–6 in tests/route-unit/routes.test.ts and the cross-process suite packages/agent-bundle/tests/worktree-proximity-journeys.test.ts still pass, and a new test proves a child sees its sibling after two agent/start events and stops seeing it after the sibling's agent/stop.