fix(trajectory): record subagent runs under their own agent_id (#33)

[justrach]

Summary

Closes #33 — /trace and any other consumer of trajectory_events
previously saw only the root agent's tool calls. Every Task dispatch
silently bypassed the recorder because the child ForgeApp constructed
inside AgentExecutor::execute had no TrajectoryRecorder attached.

This wires the raw Arc<dyn TrajectoryRepo> from ForgeAPI::chat
through to AgentExecutor via constructor injection, then builds a
child TrajectoryRecorder scoped to the dispatched agent
((conversation_id, child_agent_id, Some(parent_agent_id))). Recursive
subagent dispatches inherit the repo and build their own recorders, so
the /trace tree walks naturally for arbitrary depth.

Acceptance (from #33)

SELECT DISTINCT agent_id
FROM trajectory_events
WHERE conversation_id = ?;

now returns the parent plus every dispatched child, and child rows
carry parent_agent_id linking back to the dispatcher.

Why constructor injection (not a trait bound)

The earlier Option-A sketch added a TrajectoryRepo bound to
AgentExecutor<S>, which propagated up into ForgeServices and the
api-layer generics. Option B (this PR) keeps the trait off the type
parameters and threads a concrete Arc<dyn TrajectoryRepo> through
the same builder chain that already plumbs the recorder. Smaller blast
radius, identical behaviour.

Plumbing path

• AgentExecutor::with_trajectory_repo(...) — new builder
• AgentExecutor::execute(...) — new parent_agent_id: AgentId param;
  builds a child recorder when the repo is set
• ToolRegistry::with_trajectory_repo(...) — forwards to the
  embedded AgentExecutor
• ToolRegistry Task-tool + agent-delegation callsites — capture
  agent.id and pass through as parent_agent_id
• ForgeApp::with_trajectory_repo(...) — mirrors
  with_trajectory_recorder(...); both set together in
  ForgeAPI::chat
• ForgeAPI::chat — calls both .with_trajectory_recorder(...) and
  .with_trajectory_repo(...)

Recording stays best-effort: every recorder error is swallowed with a
tracing::warn!; a telemetry failure never aborts an agent run.

Test plan

• cargo check --workspace clean
• cargo test -p forge_app — 736 tests pass, including the new
  parent_and_child_recorders_share_conversation_trajectory
  regression test which asserts the exact acceptance criterion at
  the recorder layer (distinct agent_ids in conversation, child
  rows reference parent, root rows have no parent_agent_id)
• follow-up backport PR to release/0.1.9

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