sessions: extract local chat sessions into standalone provider

[sandy081]

Extract the local harness (in-process VS Code chat sessions) from CopilotChatSessionsProvider into a standalone LocalChatSessionsProvider under src/vs/sessions/contrib/providers/localChatSessions/.

What changed

• New LocalChatSessionsProvider (id: 'local-chat') that uses IChatService directly — no IAgentSessionsService dependency. Creates models via startNewLocalSession(), sends via sendRequest(), resolves git state via IGitService.
• Single LocalSession class with two construction paths controlled by a detail: IChatDetail | undefined parameter (new vs restored from history).
• Self-managed session list persisted to IStorageService (profile-scoped, machine target) under key sessions.localChat.sessions. Each entry stores uri, title, createdAt, lastMessageDate, workingDirectory, and optional archived flag — no runtime dependency on chat history APIs for listing.
• One-time migration from getLocalSessionHistory() brings forward existing local chat sessions on first run (guarded by sessions.localChat.migrated flag).
• Live status tracking via LocalSession.trackModel(model, onChange) — subscribes to model.requestInProgress to flip status between InProgress and Completed.
• Session type picker now hides the provider group label when no two providers offer session types with the same label, reducing visual noise.
• Pickers (mode, model, permission) extended via when clauses to match local sessions from the new provider, not just the copilot one.
• Removed local session handling from CopilotChatSessionsProvider (the LocalNewSession class, LocalSessionType, LOCAL_SESSION_ENABLED_SETTING, and associated branches in createNewSession, createNewChat, _sendFirstChat, _refreshSessionCache, etc).
• Unit tests — 10 tests covering session type declaration/hiding, workspace resolution, lifecycle, persistence, archive/unarchive, rename, deletion, status tracking from chat model.
• Spec docs: new LOCAL_CHAT_SESSIONS_PROVIDER.md and updated SESSIONS.md providers list.

Notes for reviewers

• The new provider coexists with CopilotChatSessionsProvider and AgentHost providers.
• Archiving is purely a UI/metadata concept for local sessions — no underlying chat-service archive mechanism.
• The chat session URI is stable for a local session's entire lifetime, which lets us cache and persist by URI without any commit/swap dance.
• Known limitation: the chat content viewer doesn't always open immediately after reload — being investigated separately.

[Copilot]

Pull request overview

This PR extracts “local” in-process VS Code chat sessions out of CopilotChatSessionsProvider into a dedicated LocalChatSessionsProvider (under src/vs/sessions/contrib/providers/localChatSessions/) and wires it into the Sessions/Agents window. It also updates session-type picker presentation and adjusts Copilot session actions to apply to the new local provider.

Changes:

• Introduces LocalChatSessionsProvider (id: local-chat) with storage-backed session listing, one-time migration from chat history, and live status syncing.
• Removes local-session support from CopilotChatSessionsProvider and retargets shared picker/actions when clauses to include the new local provider.
• Improves the session type picker UI by omitting provider group headers when session type labels are unique across providers; adds docs + unit tests.

Show a summary per file

File	Description
src/vs/sessions/SESSIONS.md	Documents the new local chat sessions provider and links its spec doc.
src/vs/sessions/sessions.common.main.ts	Loads the new local provider contribution in the sessions app entrypoint.
src/vs/sessions/contrib/providers/localChatSessions/browser/localChatSessionsProvider.ts	New provider implementation: session lifecycle, storage persistence, migration, and model tracking.
src/vs/sessions/contrib/providers/localChatSessions/browser/localChatSessions.contribution.ts	Registers config + provider contribution and local-specific fork handling.
src/vs/sessions/contrib/providers/localChatSessions/LOCAL_CHAT_SESSIONS_PROVIDER.md	Adds provider architecture/spec documentation.
src/vs/sessions/contrib/providers/localChatSessions/test/browser/localChatSessionsProvider.test.ts	Adds unit tests for session types, persistence, lifecycle, archive/rename/delete, and status tracking.
src/vs/sessions/contrib/providers/copilotChatSessions/browser/copilotChatSessionsProvider.ts	Removes embedded local-session implementation and related imports/branches.
src/vs/sessions/contrib/providers/copilotChatSessions/browser/copilotChatSessionsActions.ts	Retargets “local” session type references to the new provider and updates when clauses.
src/vs/sessions/contrib/providers/copilotChatSessions/browser/copilotChatSessions.contribution.ts	Removes local setting registration and local fork override (moved to local provider).
src/vs/sessions/contrib/chat/browser/sessionTypePicker.ts	Hides provider group headers/separators when session type labels are unique across providers.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 5