claude: load Agent SDK from companion extension instead of bundling (#319263)

* claude: load Agent SDK from companion extension instead of bundling

Extracts SDK loading behind IClaudeAgentSdkLoaderService with two
implementations (bundled require-based loader and a VS Code extension
loader for ms-vscode.vscode-claude-sdk), selected once at construction
by a routing service driven by experiment config. Adds install-on-demand
UX that surfaces an install button when the SDK extension is missing,
with cancellation-aware retry/fallback in the chat handler.

* claude: address PR feedback on SDK loader

- Use CLAUDE_SDK_EXTENSION_ID constant in install UX instead of hardcoded id, with placeholder in the localized error message.
- Clarify install-timeout setting description: waits for install/detection only, not activation.