[contracts] Add PlanRequest.mode field (operator-toggle frontier vs local)

[jdev-02]

Surfaced during #4 design + deferred from #5.

Why: Jon's SAR/comms-denied design — the operator should be able to override the device profile's default mode per request. SAR person on satellite -> request mode=frontier; same person in comms-denied terrain -> request mode=local.

Currently: mode is server-side (env var TERA_DEVICE_PROFILE + TERA_FRONTIER_PROVIDER). Not exposed on PlanRequest.

To add:

• Add mode: Literal['auto','frontier','local'] = 'auto' field to PlanRequest in agent/schemas.py
• Update docs/contracts/agent_routing.schema.json
• Pass req.mode through to get_registry().get(req.mode) in orchestrator
• Profile gating still applies: requesting 'frontier' in austere profile -> 403

Owners: @jdev-02 + @benschwierking (cross-lane contract change).

[kylemhicks]

Status from current main: not done yet. PlanRequest still has prompt/current/request_id/source/voice_profile, but no mode field in agent/schemas.py or docs/contracts/agent_routing.schema.json, and the ATAK/local toggle is currently handled by the Source Planner activation endpoint and agent_profile, not by the /plan contract. Keep open if we still want per-request auto/frontier/local mode in the agent contract.