Add `typecheck` script to packages/vscode-extension on main

[DrumRobot]

Context

PR #143 (spike Custom Editor, addressing #138) added "typecheck": "tsc --noEmit" to packages/vscode-extension/package.json in commit 2d10016. Running this surfaced 4 latent type errors in src/extension.ts — calls to isStale / clearStale / deletedStaleProjectCount that were never wired through in core.

main currently has the same latent bug, but pnpm -r typecheck silently skips vscode-extension because there is no typecheck script. PR #146 removes the inert calls and would resolve the typecheck error once the script is in place.

Proposed

1. Add "typecheck": "tsc --noEmit" to packages/vscode-extension/package.json on main
2. Land PR fix(vscode): remove inert stale-project cleanup calls #146 first (or together) to avoid red CI on main once the script is added

Verification

• pnpm -r typecheck runs vscode-extension and exits 0 on main
• No regression in pnpm build

Related

• Surfaced by PR feat(vscode): spike Custom Editor for JSONL preview #143 commit 2d10016
• Logic fix in PR fix(vscode): remove inert stale-project cleanup calls #146
• Sub-issue of Add Custom Editor JSONL preview for instant in-editor session viewing #138 (PR feat(vscode): spike Custom Editor for JSONL preview #143 is the spike addressing Add Custom Editor JSONL preview for instant in-editor session viewing #138)