Remove Copilot Memory (CAPI) feature (#315813)

Strip Copilot Memory (CAPI) feature entirely

Removes the CAPI-backed Copilot Memory that synced repository-scoped facts
to GitHub. The local file-based MemoryTool with user/session/repo scopes
remains as the sole memory mechanism.

- Delete AgentMemoryService and its test.
- Remove the github.copilot.chat.copilotMemory.enabled setting and its NLS string.
- Remove ConfigKey.CopilotMemoryEnabled.
- Strip all CAPI gating in memoryTool.tsx, memoryContextPrompt.tsx, tools.ts.
- Drop _dispatchRepoCAPI / _repoCreate / _sendRepoTelemetry.
- /memories/repo/ now always routes to local storage.
- Update memoryTool.spec.tsx: remove mock CAPI services and CAPI-only tests.
- Update simulationExtHostToolsService.ts for the new ToolsContribution arity.