docs(windows): add runtime architecture overview

[shailendra-nv]

Summary

Add a canonical Windows architecture overview for the native OpenShell gateway and Microsoft MXC runtime. Consolidate stable runtime and build boundaries in that page, place human build instructions in CONTRIBUTING.md, and retire the misplaced standalone MSVC build architecture page.

Related Issue

No issue required: documentation-only correction and consolidation of behavior already implemented on the windows branch; this PR changes no runtime behavior, public API, or architecture decision.

Changes

• Document the Windows gateway and in-process MXC control and data paths
• Compare ProcessContainer and IsolationSession lifecycle and enforcement semantics
• Describe create-time policy mapping, governed egress, provider credential handling, relay forwarding, ETW/OCSF auditing, and restart limitations
• Make architecture/windows.md the single canonical Windows architecture page
• Move contributor-facing Windows build and validation guidance into CONTRIBUTING.md
• Remove architecture/windows-msvc-build.md and synchronize the architecture index and Windows build skill references
• Correct the canonical overview for driver-controlled runtimes and the MXC relay configuration comment

Testing

• mise run pre-commit passes
• Agent-infrastructure inventory and stale-reference audit passes
• Unit tests added/updated (not applicable: documentation-only)
• E2E tests added/updated (not applicable: documentation-only)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[shailendra-nv]

/ok to test 63108db