Add fleet inventory command

[spyroot]

Summary

• Add a read-only fleet command that loads a YAML nodes inventory and fans out status reads with --concurrency.
• Summarize power state, health, sensor count, and max thermal reading per node through the typed read facade.
• Let fleet manage per-node connection fields from inventory, so it can run without a single global BMC target.
• Document the command and add GB300 corpus-backed offline coverage, including per-node failure isolation.

Validation

• conda run -n idrac_ctl pytest -q tests/test_fleet_inventory.py -> 2 passed in 1.45s
• conda run -n idrac_ctl pytest -q -> 817 passed, 57 skipped in 27.53s
• conda run -n idrac_ctl ruff check redfish_ctl/__init__.py redfish_ctl/fleet/__init__.py redfish_ctl/fleet/cmd_fleet.py redfish_ctl/idrac_shared.py redfish_ctl/redfish_main.py tests/test_fleet_inventory.py -> All checks passed!
• python -B -m redfish_ctl.redfish_main fleet --help rendered the fleet options.

[spyroot]

Reviewed and approved on the merits — read-only fan-out over the typed facade (tests assert GET-only), new command follows the standard registration pattern, credentials come from the inventory/env and aren't emitted. Can't merge as-is: the global query flags landed on main first, so this now conflicts on redfish_ctl/redfish_main.py (both insert just after LOCAL_COMMANDS) and docs/commands.md. Please rebase onto main and re-place FLEET_COMMANDS/is_fleet_command below the new query section, plus the fleet docs row. Clean merge after that.