feat: move VS Code behind app server

[oratis]

Summary

• run the VS Code extension through the shared protocol client and an isolated app-server child process
• preserve canonical threads, structured events, approvals, user input, interruption, and resume semantics in the editor
• build and package a real extension/app-server VSIX while keeping provider and credential code out of the extension host

Validation

• pnpm format:check
• pnpm lint (one pre-existing warning)
• pnpm typecheck
• pnpm test (960 passed, 12 skipped)
• pnpm build
• pnpm docs:check
• packaged VSIX: 8 files, 72.24 KB
• syntax checked both bundles and verified the extension bundle contains no provider, credential resolver, API-key, or RuntimeHost implementation

Stack

Depends on #195.

[oratis]

Review: approved ✅

Largest PR in the stack (+2633) and it completes the thin-client migration: desktop (#192), LSP (#195), and now VS Code all speak the same protocol to the same app-server.

What I checked

• Provider and credential code stay out of the extension host, and the app-server runs as an isolated child process. VS Code extensions share a host process with every other installed extension — keeping credentials out of that address space is the right threat model, and it's a materially stronger position than the pre-Runtime safety: mandatory policy and real cancellation #181 state where this extension could skip the permission gate entirely.
• Canonical threads, structured events, approvals, user input, interruption, and resume all preserved in the editor. This is the acceptance bar that matters: a thin client that loses interruption or resume is a downgrade users would feel immediately.
• A real VSIX is built and packaged, not just typechecked. Extension packaging fails in ways compilation doesn't (missing files, wrong entry point, activation events), so exercising the real artifact is the only meaningful check — and ci: enforce app-server release gates #208 turns it into a gate.

Consuming the shared client from #194 rather than a bespoke implementation is what keeps this reviewable at this size — most of the line count is protocol wiring and tests, not new policy.

Validation: CI green; VS Code 12 tests pass locally at the stack tip.