feat(agent-core): add session management tools

[WinterYukky]

Summary

Adds five session-management tools to the agent-core tool registry, giving
agents first-class control over the session lifecycle and hierarchy.

Motivation

Multi-agent workflows need to inspect and manage the session tree at runtime:
complete a finished session, list children/descendants, move a session under
a new parent, and export diagnostics for debugging. These tools expose that
capability to the model with appropriate permission checks.

Changes

• completeSession(sessionId) — mark a session complete and stop its worker.
  Top-level self-completion is guarded by a confirmation prompt; child sessions
  may only complete themselves.
• confirmCompleteSession — confirm a guarded completeSession call (registered
  as a required tool).
• listSessions — list sessions by scope (children / descendants / all)
  with an optional status filter (all / active / completed) and
  parent/self/top-level permission checks.
• reparentSession — move a session under a new parent, with cycle detection
  and permission checks.
• exportSessionDiagnostics — dump session metadata and message history to
  local JSON for debugging, optionally across the descendant tree.
• Register the tools in tools/index.ts (confirmCompleteSession in requiredTools;
  the rest in optionalTools).

completeSession / confirmCompleteSession set agentStatus='completed', emit an
agentStatusUpdate webapp event, and stop the worker instance.

Testing

• npm run build -w packages/agent-core (tsc) — pass
• vitest run for the new tools — 31 passed (completeSession 9, confirmCompleteSession 5, listSessions 17)
• full agent-core suite — 299 passed
• prettier format:check (agent-core) — pass

Notes

• reparentSession / listSessions / exportSessionDiagnostics build on session
  helpers already in lib/sessions (getChildSessions, getDescendantSessions,
  reparentSessions, getAllSessionsIncludingChildren).
• A related confirmCreateAgent gate is intentionally left for a follow-up PR:
  it depends on the sub-agent (parentAgentId) evolution of the agent-management
  tools, which is not yet upstream.

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