feat(agent): expose parent session identity to child extension contexts for RLM trees

[rynfar]

Part of #22 (Meridian reliability umbrella). Prerequisite for rynfar/meridian#902 (proxy-side parent-tree cancellation).

Problem

After #23 / PR #31, every RLM child stamps its own session identity via its own extension runner. But the proxy still cannot associate a child with its parent, so aborting a parent's request cannot tear down the children's provider streams proxy-side. The wire contract proposed in #23 is an additive field in the metadata.user_id JSON envelope: {"session_id": "<child>", "parent_session_id": "<root>"}.

The extension cannot produce that today: a child runner's ctx.sessionManager.getSessionId() returns the child's id, and nothing extension-visible names the parent.

Proposed shape

• Expose the spawning session's id to a child session's extension context — e.g. ctx.sessionManager.getParentSessionId?(): string | undefined on the read-only view (undefined for root sessions), or an equivalent context field. Must work for inline and daemon RLM children and compose with the sessionIdScope mechanism from PR fix(coding-agent): scope provider identity to the agent that owns each request #31 (a scoped view keeps reporting the owner's parent).
• Additive only: existing extensions unchanged; no daemon wire change (the value rides inside the payload extensions already produce).
• Update packages/coding-agent/docs/extensions.md.

Acceptance

• With a test extension: an inline RLM child's before_provider_request can read both its own session id and its parent's; a root session reads undefined; a side question / auxiliary scoped call reports its owner's parent linkage consistently.
• Focused tests at the runner/context seam.