[Tracking] Webview→native VS Code migration: architecture review & cleanup

[sydneyrenee]

What we are supposed to be doing

Per CHAT_MIGRATION_PLAN.md, docs/migration/, docs/dynamic-models-*.md, and src/ux/README.md, the goal of the retire-webview effort is:

1. Retire the webview/React UI and the end-of-life @vscode/webview-ui-toolkit.
2. Express every interaction through native VS Code APIs — Chat Participant (vscode.chat.createChatParticipant), TreeView sidebar, QuickPick/InputBox, status bar, command buttons — instead of a custom message bus + HTML/CSP.
3. Keep the neutral provider layer (provider-agnostic handlers + neutral history) with dynamic model fetching so model lists come from provider APIs rather than hardcoded maps.
4. Migrate tests to Mocha (Microsoft-supported, runs in real VS Code/Electron).

Where we actually are (architecture review findings)

The native surface (src/ux/) + TaskManager is wired and active; extension.ts no longer registers a webview. But the migration is half-applied, leaving parallel/dead systems and a few live bugs:

• Sidebar menu items never render: view id mismatch (sidebarView vs SidebarProvider) #244 — sidebar menu items never render (view id mismatch) — live bug, trivial fix
• Dynamic-models subsystem is orphaned: buildApiHandler never uses ProviderFactory / ModelRegistry / Dynamic*Handler #245 — dynamic-models subsystem orphaned (buildApiHandler never uses ProviderFactory/ModelRegistry/Dynamic*Handler) — goal Added rebranding support, split ClineProvider.ts, other changes #3 not actually wired
• Remove dead webview layer (src/core/webview) still imported across the codebase #246 — remove dead webview layer (src/core/webview/) still imported everywhere
• TheaTask depends on the retired TheaProvider shape via a no-op shim (abstraction inversion) #247 — TheaTask depends on retired TheaProvider shape via no-op shim; chat drops partial (no streaming)
• i18n: fire-and-forget init race + locale resources may fail to load #248 — i18n fire-and-forget init race + locale path
• Add regression guard for empty default model ids (systemic crash class) #249 — regression guard for empty default model ids (systemic crash class, fixed)

Net assessment

Two strategic decisions drive the rest:

• Dynamic-models subsystem is orphaned: buildApiHandler never uses ProviderFactory / ModelRegistry / Dynamic*Handler #245: finish wiring the dynamic-model subsystem into buildApiHandler, or delete it. Today ~40 files of parallel provider/registry/webview infra exist that the running product never calls.
• TheaTask depends on the retired TheaProvider shape via a no-op shim (abstraction inversion) #247/Remove dead webview layer (src/core/webview) still imported across the codebase #246: decouple TheaTask from TheaProvider, then delete the webview layer.

Test status on branch: npm run test:unit = 174 passing / 0 failing (Mocha).