[Routine - VT] fix(extension): refresh treeView description after workspace folder changes

[winterdrive]

1. Pre-flight Check

Checked open PRs and active routine/* branches before selecting a target (see full list below). This PR touches only src/extension.ts, which is not touched by any currently open PR:

PR	Files touched
#114	src/core/ProjectExplorer.ts, src/test/unit/projectExplorerMaxResults.test.ts
#113	src/sendTo.ts
#112	src/core/FileEntryMatcher.ts, src/test/unit/fileEntryMatcher.test.ts
#111	src/core/GroupManager.ts, src/test/unit/groupManagerNonArrayConfig.test.ts
#110	mcp-server/package*.json, package*.json (dependency bump only)
#109	src/mcp/SkillGenerator.ts
#108	src/dragAndDrop.ts, src/test/unit/dragIsDescendantCycleGuard.test.ts
#107	src/core/AutoGrouper.ts, src/test/unit/autoGrouperBookmarks.test.ts
#106	i18n/*.json, src/provider.ts
#105	src/commands.ts, src/i18n.ts, src/test/unit/copyGroupName.test.ts
#104	mcp-server/src/server.ts

No open PR or active routine branch touches src/extension.ts, so this change does not overlap with any in-flight work.

2. Changes

onDidChangeWorkspaceFolders in src/extension.ts calls provider.reinitializeScopes(), which prunes activeScopeIds entries for any scope whose workspace folder was just removed. However the TreeView.description string (set once at activate() time from provider.computeScopeDescription()) was never recomputed after that prune, so the panel header could keep showing a stale scope label/count (e.g. still say "2 scopes" after one of the active scopes' folders was removed from the workspace).

Fix: recompute and re-assign treeView.description right after reinitializeScopes(), mirroring the same pattern already used in src/commands.ts for the scope-selection commands.

This PR does not modify any VS Code command registrations, contributed configuration keys (package.json), dependencies, or the tab-group serialization/storage format. It is a 3-line, single-file change.

3. Safety Verification

Commands run locally, all passed:

• npx tsc -p ./ — no type errors
• npm run test (tsc -p ./ && jest --runInBand) — 31 test suites / 203 tests passed

No lint or format:check script exists in package.json, so those steps were skipped.

4. CI / Release Gate Note

This is a daily routine Draft PR. Package version bump and CHANGELOG.md updates are intentionally deferred to the weekend release/integration PR. If CI fails solely due to the repository's version-bump gate, that is expected release-readiness behavior for a routine PR, not a code validation failure.