local → agent host migration: In‑place switch, imported session fidelity

[vijayupadya]

Description

Improves the experience of continuing a local chat in an agent‑host session (Copilot CLI), so it feels like the same conversation continuing rather than jumping into a brand‑new, different-looking session.

What changed

In‑place switch (no jarring navigation)

• Editor path: replace the source chat editor in place instead of opening a new tab (replaceEditor: !isSidebar); falls back to opening a new editor if there's no active chat editor, so a pending send is never lost.
• Sidebar path: suppress the chat Sessions list for the whole migration transition (load → materializing send → untitled→real rebind), so it no longer flashes the session list mid‑switch.
• Show the chat view's working indicator for the duration of that transition so the (unavoidable) backend‑materialization gap doesn't look hung.

Imported session reads like a normal session

• Generate a title from the imported conversation (reusing the existing fork‑title path), so migrated sessions get clean titles like any other.
• Imported inline file/symbol references now show short labels (file basename / symbol name) instead of leaking full workspace‑relative paths.

[Copilot]

Pull request overview

Improves local-chat migration into Agent Host sessions while preserving conversation fidelity and reducing disruptive UI transitions.

Changes:

• Replaces source chat editors in place and suppresses transient sidebar UI.
• Adds preparation progress and generated imported-session titles.
• Shortens imported file and symbol reference labels.

Show a summary per file

File	Description
chatContinueInAction.ts	Requests in-place editor migration.
chatSessions.contribution.ts	Manages replacement, suppression, and progress.
chatViewPane.ts	Adds session-list suppression support.
agentHostSessionHandler.ts	Shows delayed preparation status.
agentService.ts	Generates imported-session titles.
importLocalConversationToAgentSession.ts	Shortens imported reference labels.
importLocalConversationToAgentSession.test.ts	Tests reference-label conversion.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 4
• Review effort level: Medium