refactor: use Dynamo session identity

[ishandhanani]

Summary

Pi and Hermes now emit Dynamo's canonical session headers and session-based trace context with explicit subagent lineage. This keeps identity passive while aligning agent plugins with ai-dynamo/dynamo#10896.

How This Was Implemented

• Renamed Pi headers, environment variables, trace fields, and public helpers from trajectory to session terminology.
• Preserved root/child lineage through session_id and parent_session_id, including self-parent rejection for nested agents.
• Removed Pi's generic session-affinity header switch so identity alone does not activate sticky routing.
• Renamed the Hermes plugin and CI coverage to the session-only contract.

Walkthrough

• pi-plugin/src/light/session.ts: derives root and child session identity from Pi and pi-subagents metadata.
• pi-plugin/src/light/provider.ts: emits canonical Dynamo session headers without enabling affinity.
• pi-plugin/src/light/tool-relay.ts: publishes session-based Dynamo tool-event context.
• hermes-plugin: maps Hermes session_id to x-dynamo-session-id.

Validation

• All completed test-plan items below passed locally.

Test plan

• cd pi-plugin && npm run check
• cd pi-plugin && npm test — 4 passed.
• cd pi-plugin && npm run build
• python3 -m unittest discover -s hermes-plugin/tests
• Run the integration smoke against refactor(agents): unify identity on session IDs dynamo#10896.