Define the microVM environment architecture and path contract

Parent: #526

Outcome

The microVM feature has one explicit architectural contract for path identity, trust boundaries, package/module boundaries, environment profiles, lifecycle, and deferred identity capabilities before implementation fans out.

Scope

• Add the new ADR covering the external local microvmd, nested module boundary, host/guest responsibilities, session-lifetime environment, and no-local-fallback rule.
• Define the three path roles precisely:
  • source checkout supplied by the local client;
  • prepared host session worktree persisted as the session Workspace;
  • guest execution root /workspace returned by the live Environment Workspace.
• Define which project artifacts are discovered host-side from the prepared worktree and which user-global artifacts remain outside the VM.
• Define skill-asset materialization/read-only mounting.
• Define environment_profile independently from the existing tool-surface profile.
• Define the local trusted-UID identity mode and the reserved #452-compatible future principal assertion.
• Define supported first-release platforms and explicit deferrals: schedules, remote placement, multi-user enforcement, cross-principal sharing.
• Update ADR 0027 resource and rehydration inventories for every daemon, VM, cache, socket, stream, registry, worktree, reconciler, and lifecycle record that outlives a call.

Acceptance

• Every persisted and live path field has one non-overlapping meaning.
• The agent-facing tool path never receives or requires the host source/worktree path.
• No heavy microVM dependency enters engine/ or the root module's default build.
• The trust boundary clearly distinguishes guest egress from host service egress.
• Deferred jhrozek identity issues are named with their exact trigger and do not block local phase 1.
• Architecture, implementation notes, usage, user docs, and generated llms.txt remain consistent.