Introduce pluggable fleet storage

[floitsch]

Summary

• add a FleetStore persistence strategy and a FileFleetStore for the existing fleet.json/devices.json layout
• give Fleet the store and move group/device mutations out of command handlers
• collapse FleetWithDevices into Fleet while retaining explicit reference-only and device-required construction paths
• add a shared contract test that exercises both an in-memory store and the file-backed implementation

Compatibility

This is a behavior-preserving extraction. Existing fleet files, reference files, migration fields, server wiring, recovery URLs, and pod specification locations remain unchanged. Server and migration fields on FleetStore are explicitly transitional until workspace construction supplies backend strategies independently.

Tests

• toit analyze src/cli/cli.toit
• toit analyze --project-root tests tests/*-test.toit
• toit run --project-root tests tests/fleet-store-test.toit
• cmd-fleet-group-test.toit with local HTTP servers
• cmd-fleet-create-reference-test.toit with local HTTP servers
• fleet-root-test.toit with local HTTP servers