feat: PicoClaw worker spawner and extended CC session events

[DarlingtonDeveloper]

Summary

• Adds process.Spawner that subscribes to swarm.task.*.assigned, filters for runtime: "picoclaw", and spawns picoclaw worker processes with mission directory, briefing JSON, and timeout management
• Adds PicoClawConfig to config with defaults (binary, mission_base_dir, default_timeout=5m, max_concurrent=20)
• Extends CCSessionCompletedData with backwards-compatible Model, Runtime, and token count fields emitted by PicoClaw workers
• Adds SubjectAllTaskAssigned wildcard subject for task assignment subscription
• Wires spawner into orchestrator main alongside process subscriber

New files

File	Purpose
internal/process/spawner.go	Task assignment subscriber, picoclaw process lifecycle management
internal/process/spawner_test.go	Runtime filtering, concurrency limit, taskAssignment marshal tests
internal/config/config_picoclaw_test.go	PicoClaw config defaults and YAML parsing
internal/hermes/picoclaw_test.go	Extended CC event fields, backwards compat, subject constant

Test plan

• go build ./... — clean
• go vet ./... — clean
• All existing tests pass
• New tests: config defaults (2), hermes event fields + backwards compat (4), spawner runtime filter + concurrency limit + serialization (5)
• Integration: assign a picoclaw-runtime task via NATS, verify worker spawns and mission directory created

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