Expose root lineage for OpenCode agent sessions

[achangkuon]

Herdr currently exposes an agent session reference, but root-aware integrations cannot tell whether the current OpenCode session is top-level or a native child, or bind lineage to the exact pane and terminal. Guessing can preserve stale or child state as root.

Could Herdr provide an additive, versioned machine-readable root-lineage observation or capability that identifies the pane, terminal, and current OpenCode session, carries nullable parent lineage, and invalidates fail-closed when the route or session changes? Existing agent_session behavior should remain compatible.

This would let external orchestrators avoid clobbering child sessions, project only observed root state, and negotiate compatibility across Herdr upgrades.