Support Agent Host session attachments with Copilot CLI events.jsonl

[anthonykim1]

Part of: #315193

• Scope Agent Host session-reference attachments to Copilot CLI sessions that resolve to a host-local events.jsonl.
• Keep the visible Sessions chip by converting supported sessionReference variables into a small Simple attachment with replay metadata.
• Attach the full Copilot CLI trajectory as a hidden Resource attachment pointing at events.jsonl, instead of packing transcript text into Simple.modelRepresentation.
• Filter the Agent Host Sessions... picker to only show attachable Copilot CLI sessions when the current chat is Agent Host-backed.
• Preserve normal/local/EH harness #session: completion behavior; this PR does not change non-Agent-Host session attachment behavior.
• Keep hidden trajectory resources out of replay UI so reload/resume restores only the visible session chip.

Supported session matrix:

Referenced session	Example URI	Has events.jsonl path?	This PR behavior
EH Copilot CLI	copilotcli:/<id>	Yes	Attach visible session chip + hidden trajectory file
Local Agent Host Copilot CLI	agent-host-copilotcli:/<id>	Yes	Attach visible session chip + hidden trajectory file
Remote Agent Host Copilot CLI	remote-<authority>-copilotcli:/<id>	Yes, when connection/home resolves	Attach visible session chip + hidden trajectory file
Non-Copilot-CLI Agent Host sessions	provider-specific AH URI	No shared events.jsonl contract	Not supported by this PR
Claude / local / other chat sessions	provider-specific URI	No shared events.jsonl contract	Not shown in Agent Host attach picker and not converted as model context

---

Inspirations from:

• Copilot CLI session URI to events.jsonl resolution comes from buildHostLocalEventsPath.
• Existing Agent Host Copilot attachment forwarding maps Simple.modelRepresentation to a text/plain SDK blob and Resource attachments to SDK file/directory attachments in _toSdkAttachment.
• Replay behavior restores visible simple attachments and skips hidden trajectory resources in messageAttachmentToVariableEntry.

[Copilot]

Pull request overview

Enables attaching session references for the agent-host Copilot flow by round-tripping session-reference variables through agent-host “simple” attachments, and enriching those attachments with referenced session transcript content when available.

Changes:

• Add a session-reference attachment format (displayKind + metadata) and restoration helper.
• Update agent-host variable→attachment conversion to hydrate referenced session transcripts via IChatSessionsService, with a debug-events fallback via IChatDebugService.
• Expand session picker filtering to include agent-host Copilot sessions and add targeted tests for restore/convert paths.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts	Adds tests for restoring session-reference attachments and for converting sessionReference variables into agent-host simple attachments (including transcript hydration scenarios).
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/stateToProgressAdapter.ts	Restores session-reference variable entries from simple attachments during state→variable reconstruction.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionReferenceAttachment.ts	New helper module defining the session-reference attachment shape, metadata, transcript rendering helpers, and restoration logic.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts	Hydrates and serializes session-reference variables into attachments, including transcript embedding from session history or debug events.
src/vs/workbench/contrib/chat/browser/actions/chatContext.ts	Includes AgentHostCopilot in the session provider filter for the session reference picker.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 3
• Review effort level: Low