feat(sandbox): add Tier 2 native execution fabric

[haelyra]

Problem and behavior

Native platform testing and multi-job execution need stronger isolation, explicit placement, bounded resources, and evidence that survives worker loss. This PR adds Tier 2 VM and hosted CI adapters plus an opt-in version 2 execution-fabric envelope that separates execution class from placement and states its shell-only coverage.

Plans and receipts now bind route, class, placement, operator, included and excluded surfaces, resource decisions, credentials, immutable environments, artifacts, trajectories, cleanup, and promotion. Missing telemetry is disclosed; stale telemetry or a measured limit breach stops the job and triggers owned-workspace cleanup.

Scope

• Lume, Lima, optional Tart, Windows-to-CI, Microsandbox, and hosted CI routes
• Linux, macOS, and Windows hosted matrix with aggregate evidence validation
• execution class, placement, shell-only coverage, and resource-monitoring v2 schemas
• isolated copy/worktree scheduling, credential leases, immutable artifacts, evaluation, and candidate-only promotion
• manual native Lume acceptance on a dedicated Apple Silicon runner

Validation

• final immutable tree passes 4,797/4,797 repository tests
• fast sandbox, fabric fault, ESLint, Markdown, workflow-security, YAML, publish-surface, and diff gates pass
• exact-head hosted fast, capability-probe, and native matrix workflows pass on Linux x86_64, macOS arm64, and Windows x86_64
• live Tier 0 execution-fabric demo passed worktree, patch, evaluation, trajectory, and cleanup
• retained real Apple Silicon Lume evidence verifies guest cleanup; current host preflight safely denies a new launch because 4.73 GiB is available versus 9.00 GiB required

Boundaries

This M2 slice governs shell commands and their child processes. Whole-agent containment and the M3 ExecutionCapsule remain future contracts. Native Tier 2 requires an admitted host; unavailable local platforms return a truthful hosted route or block.

Stack

This is PR 3 of 3. It targets and depends on #3035 so its diff contains only Tier 2, hosted CI, and execution-fabric behavior.

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