feat(devices): answer when a device's current boot began

[thymikee]

Summary

Adds one question to the platform boundary — when did this device's current boot begin — as @agent-device/contracts/device-boot. An answered probe returns host-clock bootedAtMs; a device family with no honest answer returns unsupported-device rather than a guess.

An Apple Simulator reports the start time of the launchd_sim process naming that device, rejecting a stamp that lies in the future. Android converts /proc/uptime on the host clock, bounded by the instant the probe began so response latency cannot move the answer forward, and a guest clock that disagrees with the host cannot move it at all. src/platform-runtime-device-boot.ts is the root hub that dispatches by family; nothing consumes it yet.

Part 2 of 3 for #2538, on top of #2574. 489 gross lines.

Validation

Tested at 6fae37be00. pnpm check:layering passes, which enumerates the two new package subpaths and the contracts export. pnpm check:fallow --base origin/main reports no issues. Typecheck and lint pass. pnpm check:package imports all 13 published entry points from a clean install and runs the published CLI.

The probes are host-process reads on the same commands the platform packages already use for boot state, so no new device behavior is exercised here; #2570 drives them against a live simulator and emulator reboot.

[github-actions]

Size Report

Metric	Base	Current	Diff
Installed (including dependencies)	4.52 MB	4.52 MB	0 B
Package (unpacked)	4.52 MB	4.52 MB	0 B
Package (download)	1.33 MB	1.33 MB	-1 B

Startup median (7 runs, lower is better):

Scenario	Base	Current	Diff
CLI --version	29.1 ms	29.1 ms	-0.0 ms
CLI --help	80.1 ms	79.2 ms	-0.9 ms

[thymikee]

No findings at 6fae37b. The Android probe now uses a conservative pre-request timestamp, so response latency cannot falsely invalidate a newer claim. Checks are green; ready for human review after #2574. The consuming takeover behavior remains in #2570.