Freeze fs on old-envd guests via the envd exec API

[bchalios]

A filesystem-only pause must quiesce the guest rootfs before snapshotting to close the sync->pause write race. Today only envd >= 0.6.6 (native /fsfreeze) does a real FIFREEZE; older guests fall back to a plain sync, which does not block writes and can capture a torn ext4 journal.

For those guests, when the fsfreeze-via-exec flag is on, run fsfreeze -f / through the envd process API instead — the same FIFREEZE, no native endpoint needed, running entirely inside the guest (no host attack surface). Falls back to sync per-pause if the guest lacks the fsfreeze binary or the freeze fails. On the pause-failure rollback the rootfs is thawed via the same API (bounded, best-effort).

Gated behind featureflags.FsFreezeViaExecFlag (off by default).

[cursor]

PR Summary

Medium Risk
Changes mandatory pre-pause filesystem quiescing on the filesystem-only pause path; a failed or timed-out exec thaw could leave a live sandbox with a frozen rootfs until teardown, though behavior is flag-gated and defaults unchanged.

Overview
Filesystem-only pause on guests without native /fsfreeze can use fsfreeze-via-exec (off by default) to run fsfreeze -f / through the envd shell API instead of only sync, so the rootfs is quiesced like FIFREEZE rather than leaving the sync→pause write race open. The flow probes for fsfreeze with command -v (missing binary or probe errors still fall back to sync); freeze failures abort the pause without syncing onto a possibly frozen filesystem, and cleanup runs a bounded best-effort fsfreeze -u / thaw. Guest shell steps are centralized in runGuestShellCommand, and pause telemetry records quiesce method (fsfreeze, fsfreeze-exec, or sync).

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[codecov]

Codecov Report

❌ Patch coverage is 13.09524% with 73 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/orchestrator/pkg/sandbox/reclaim.go	13.09%	73 Missing ⚠️

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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