fix(llm): honor per-device 'available' flag in device validation

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Before this fix, a REQUIRED_HARDWARE check could pass on hardware that isn't actually usable: LemonadeManager treated the mere presence of a device key in get_system_info()'s devices dict as "detected." A device reported with available: false — e.g. an NPU that's present but disabled or unpowered — was misread as detected, so an agent requiring that device would proceed against hardware it can't use. Now detection keys off the per-device available boolean (the documented get_system_info() contract), so present-but-unavailable devices fail the requirement with the existing actionable device error.

A missing available flag is still treated as available, preserving behavior for sysinfo payloads that omit it. The device-error message contract is unchanged, so the blast radius across agents that set REQUIRED_HARDWARE stays behavior-compatible except for the disabled-device case this fixes.

Closes #2079

Test plan

• python -m pytest tests/unit/test_multi_device_wiring.py -x passes
• python util/lint.py --all passes
• New TestPerDeviceAvailableFlag cases cover: available: true passes, available: false fails with the gaia init --profile npu remedy, key-absent fails, missing-flag treated as available

🔍 Technical details

Root cause: _validate_device_requirement built its detected set from set(devices.keys()) (src/gaia/llm/lemonade_manager.py), ignoring each entry's available boolean.

Changes:

• Added module-level _device_is_available(info) helper — truthy available for a dict entry, any(...) over a list entry (amd_dgpu can be a list), and a missing flag treated as available for backwards compatibility.
• The dict branch now builds detected via {name for name, info in devices.items() if _device_is_available(info)}; the list-of-dicts branch skips entries that aren't available before extracting the device type.
• Added TestPerDeviceAvailableFlag in tests/unit/test_multi_device_wiring.py covering the three acceptance cases plus the missing-flag backwards-compat case.

Verified: tests/unit/test_multi_device_wiring.py device classes, test_npu_device_support.py, and test_hardware_selection.py all pass (44 device tests); python util/lint.py --black --isort --flake8 --pylint clean. The import-validation lint step and one unrelated fastapi-dependent test in this file fail only because optional extras aren't installed in this environment — neither is touched by this change.