docs(architecture): reset subsystem docs

[drew]

Still WIP

Summary

Architecture docs have become unwieldy. Reset the architecture directory around seven canonical subsystem docs and a new high-level architecture README. The new docs use concise maintainer narratives with Mermaid overview diagrams instead of long feature-specific source maps.

Related Issue

None.

Changes

• Replaced the architecture docs sprawl with README.md, client.md, supervisor.md, sandbox.md, policy.md, gateway.md, providers.md, and build.md.
• Updated AGENTS.md with subsystem ownership and architecture doc guidance for future agents.
• Updated stale references in agent skills, examples, VM docs/scripts, and Mermaid lint comments to point at the new canonical docs.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

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

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[drew]

• review skills and make sure we aren't recommending doc updates. this should happen through AGENTS.md
• talk about driver philosophy and how we want to integrate with the ecosystem