[Bug] Session rename does not persist in /resume picker

Bug Description
Session rename does not persist in /resume picker - sessions unidentifiable

Problem:
The /resume picker displays sessions using firstPrompt (first user message) rather than custom names set via /rename.
This makes sessions nearly impossible to identify and resume.

Steps to reproduce:

1. Start a session with any initial message (e.g., 'yes' after a continuation prompt)
2. Work on a substantial task across multiple hours/days
3. Run /rename with a custom name - get confirmation
4. Exit session
5. Run /resume - session still shows as original first message, not the custom name

Expected behavior:
Custom session names should:

1. Persist in sessions-index.json (currently has no name field)
2. Display in the /resume picker instead of firstPrompt
3. Survive session restarts and context compactions

Actual behavior:

• /rename appears to work in-session but name is not stored
• sessions-index.json only stores firstPrompt and summary
• Picker shows unhelpful text like 'yes', 'No prompt', or truncated first messages

User impact - HIGH:

• Cannot identify which session contains which work
• Forced to start new sessions, losing valuable context
• Hours of accumulated context and conversation history become inaccessible
• The /rename feature is effectively non-functional for its intended purpose

Suggested fix:
Add name field to sessions-index.json schema. Picker should display: name (if set) -> summary (fallback) ->
firstPrompt (last resort)

Environment Info

• Platform: win32
• Terminal: null
• Version: 2.1.31
• Feedback ID: 7b41175c-eb2e-43e9-89e2-fa275e2c9a23

Errors

[{"error":"Error\n    at uh (B:/~BUN/root/claude.exe:42:1144)\n    at <anonymous> (B:/~BUN/root/claude.exe:47:10089)\n    at emit (node:events:92:22)\n    at endReadableNT (internal:streams/readable:861:50)\n    at processTicksAndRejections (native:7:39)\n    at request (B:/~BUN/root/claude.exe:49:2149)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-02-05T05:53:48.228Z"},{"error":"Error: EPERM: operation not permitted, symlink 'C:\\Users\\ulfma\\.claude\\projects\\C--Users-ulfma-Projects\\d6669418-6eaf-4c07-a135-13a6e48afabb\\subagents\\agent-a26c59f.jsonl' -> 'C:\\Users\\ulfma\\AppData\\Local\\Temp\\claude\\C--Users-ulfma-Projects\\tasks\\a26c59f.output'\n    at symlinkSync (unknown)\n    at DGH (B:/~BUN/root/claude.exe:1494:1154)\n    at DtD (B:/~BUN/root/claude.exe:1528:1268)\n    at <anonymous> (B:/~BUN/root/claude.exe:2895:922)\n    at <anonymous> (B:/~BUN/root/claude.exe:2895:1840)\n    at run (node:async_hooks:62:22)\n    at CUH (B:/~BUN/root/claude.exe:217:14675)\n    at call (B:/~BUN/root/claude.exe:2895:616)\n    at async fME (B:/~BUN/root/claude.exe:3103:13642)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-02-05T05:57:28.455Z"}]