feat: add Atomic Source Control provider

[vinceblock99]

Summary

• add an Atomic Source Control provider for VS Code, including multi-root workspace support
• show modified, added, deleted, untracked, and conflicted resources with working-copy and Quick Diff views
• add, record, restore, refresh, and switch views without shell interpolation
• validate the versioned CLI JSON protocol and provide an actionable upgrade hint for older clients
• serialize CLI access around Atomic's exclusive database lock and prevent self-triggered refresh loops
• discover repositories created after startup, clear stale SCM state on protocol failures, and guard restore paths against symlinks
• package a minimal VSIX and verify it in CI

Requires atomicdotdev/atomic#188.

Closes atomicdotdev/atomic#171.

add

restore

Test Coverage

• npm test — 14 unit and integration-boundary tests passed
• npm run package — VSIX packaged successfully
• live cross-repository test against the latest Atomic CLI exercised concurrent status, view list, and pristine-content requests
• reproduced Atomic's database-lock behavior and verified per-repository command serialization

Pre-Landing Review

Independent security, maintainability, testing, API-contract, and adversarial reviews completed. All merge-blocking findings were fixed; final review reported no findings.

Design Review

No web UI files changed. The extension uses VS Code's native Source Control API and theme icons.

Plan Completion

No plan file detected.

Documentation

• README.md documents Source Control features, required CLI JSON commands, usage, configuration, refresh behavior, development workflow, and schema compatibility.

Test plan

• recognize Atomic repositories at startup and after atomic init
• render SCM resource groups and current view
• open modified/deleted/added diffs with the correct baseline
• pass paths, messages, and view names as safe process arguments
• reject incompatible JSON schemas and unsafe paths
• serialize concurrent CLI operations
• build and package the installable VSIX