Track full session porting for Continue in Agent Host

[anthonykim1]

Related: #315193
Related attachment tracking issue: #323969
Related scoped PR: #323773

Context

PR #323773 handles a narrow attachment case: when a user attaches a Copilot CLI session from Agent Host chat, Agent Host can forward the session's events.jsonl as context.

This issue tracks a different problem: full session porting for continue in flows. In this path, the user is not merely attaching a previous session via the +/context UI. Instead, the old session should become, or be imported into, the active Agent Host conversation.

Attachment vs porting

Flow	User intent	Mechanism	Current PR #323773?
Session attachment	Reference another session as context in the current chat	Visible session chip plus model-readable attachment, currently events.jsonl for Copilot CLI	Yes, only for Copilot CLI events.jsonl sessions
Full session port / continue in	Continue/resume an existing session in Agent Host	Convert/import/resume the previous session as the active Agent Host conversation	No

Porting questions to resolve

• Which source session types should be portable into Agent Host?
• What is the canonical source of truth for each source session type?
  • Copilot CLI events.jsonl
  • IChatModel / workbench chat model state
  • provider-specific session content
  • AHP session state
• How should tool calls, tool results, file edits, approvals, working directory, model selection, and session metadata map into the target Agent Host session?
• Should porting preserve original session identity or create a new Agent Host session linked back to the source?
• What should happen when source session data is partial, provider-specific, or unavailable?

Initial matrix

Source session type	Example URI	Possible source of truth	Porting status
EH Copilot CLI	copilotcli:/<id>	~/.copilot/session-state/<id>/events.jsonl	Needs design
Local Agent Host Copilot CLI	agent-host-copilotcli:/<id>	AHP state plus Copilot CLI events.jsonl	Needs design
Remote Agent Host Copilot CLI	remote-<authority>-copilotcli:/<id>	Remote AHP state plus remote events.jsonl	Needs design
Other Agent Host/AHP sessions	provider-specific Agent Host session URI	AHP state/provider state	Needs design
Local/workbench chat sessions	workbench chat session URI	IChatModel / chat session model	Needs design
Claude Code / other providers	provider-specific URI	Provider-specific session state	Needs design

Non-goal

This issue does not track using a previous session as an attachment/context item. That matrix is tracked in #323969.

/cc @connor4312 @roblourens