W-22203667: add timestamp, session type, and index to agent preview sessions output

[andresrivas-sf]

Summary

• Adds timestamp (ISO string) and sessionType (simulated/live/published) to session-meta.json written on agent preview start
• Introduces index.json in the sessions directory to maintain creation-ordered session metadata for easier browsing by humans and agents
• Surfaces new fields as Started At and Session Type columns in agent preview sessions table and JSON output
• listCachedSessions prefers the index for ordered results, falls back to directory scan for sessions created before this change

Test plan

• All existing unit tests pass (yarn test)
• New tests added for index write, removal, ordering, and directory-scan fallback
• Schema regenerated for agent-preview-sessions command

@W-22203667@

🤖 Generated with Claude Code

[andresrivas-sf]

The NUT failures in this run are pre-existing and unrelated to this PR's changes. The same assertion failure at test/nuts/z2.agent.publish.nut.ts:132 (expected SyntaxError: Unexpected 'syem') is also failing on PR #399 (fp/add-agent-api-name-to-preview-json) from a different author, confirming this is a flaky test in agent publish authoring-bundle error handling that predates these changes.

[WillieRuemmele]

If we're writing a file in the session history, it should be done in the library, so VSC and plugin conversations save the same data to the same locations

[WillieRuemmele]

this should be in the library, reading/writing files

[WillieRuemmele]

and then claude review:

Branch Review                                                                                                   
                                             
  Verdict: REFINE                                                                                                 
                                                                                                                  
  Summary: The core feature is well-structured and tested, but the index file introduces two real race conditions,
   a type widening inconsistency, and a pattern violation in the import style.                                    
                                                                                                                  
  ---                                                                                                             
  Critical                                                                                                        
                                                                                                                  
  - Race condition on concurrent createCache calls — src/previewSessionStore.ts:55-61 — Two parallel createCache
  calls for the same agent will both read the same stale index, append their entry, and write back; the second    
  write overwrites the first, silently dropping a session from the index. While rare in a CLI, it is a real     
  data-loss scenario for programmatic callers.                                                                    
  - Race condition on concurrent removeCache calls — src/previewSessionStore.ts:97-103 — Same read-modify-write 
  pattern; concurrent removes can restore a deleted entry or silently no-op.                                      
  
  ---                                                                                                             
  Worth Fixing                                                                                                  
                                                                                                                  
  - sessionType?: string should be sessionType?: SessionType — src/commands/agent/preview/sessions.ts:29 — The  
  result type widens the union to string, breaking the discriminated-union guarantee for JSON consumers.          
  CachedSessionInfo correctly types it as SessionType; the command result type should match.
  - Inline import() type expression instead of a top-level named import — src/commands/agent/preview/start.ts:173 
  — resolveSessionType uses : import('../../../previewSessionStore.js').SessionType as its return annotation.     
  Every other consumer in this directory uses a normal top-level named import. SessionType should be added to the
  existing import { createCache } statement on line ~20.                                                          
  - displayName = index[0]?.displayName assumes sessions are homogeneous — src/previewSessionStore.ts:200 — If  
  different sessions for the same agentId were started with different --authoring-bundle or --api-name values, the
   first session's displayName is applied to all. CachedSessionInfo doesn't carry displayName, so there's no way
  to surface per-session display names. The behavior is the same as before the diff (the original code also read  
  from sessionIds[0]), but now the assumption is more exposed.                                                  
  - Unhandled writeFile errors in index updates — src/previewSessionStore.ts:60 and :102 — If the index write
  fails (disk full, permissions), the error is swallowed silently, leaving the index out of sync with session     
  directories. The existing pattern for the meta file write (line ~51) propagates the error; the index write
  should too, or at minimum log a warning.                                                                        
  - SessionIndex exported but used only internally — src/previewSessionStore.ts:28 — readSessionIndex is not    
  exported, so no external caller can ever receive a SessionIndex value. Removing the export makes the scope      
  honest.
                                                                                                                  
  ---                                                                                                           
  Simplification Opportunities
                                                                                                                  
  - Double-read of session-meta.json in fallback path — src/previewSessionStore.ts:219-225 — The fallback
  directory-scan loop already reads and parses each session's meta file, but discards displayName from the first  
  entry. It then re-reads sessions[0]'s meta file a second time solely to recover displayName. Capturing it during
   the first pass eliminates a redundant I/O call.                                                                
                                                                                                                
  ---