[FEATURE] Show session summary in `--resume` session picker

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When using claude --resume, the session picker only displays the last user prompt
for each session. This makes it very hard to identify which session corresponds to
which task, especially when sessions are old or the last prompt was a short/generic message.

Proposed Solution

Display a short auto-generated summary (1–2 sentences) alongside each session entry
in the --resume picker — similar to how ChatGPT or other tools generate conversation titles.

This summary could be:

• Generated at session end (or periodically during the session)
• Derived from the first meaningful user message
• Or explicitly set by the user with a command like /title My session title

Alternative Solutions

_No response_

Priority

Low - Nice to have

Feature Category

CLI commands and flags

Use Case Example

Instead of:
› 2026-03-09 fix the bug with null pointer
2026-03-08 yes
2026-03-07 ok do it

Something like:
› 2026-03-09 [#904 i18n] Added missing translation keys for French locale
2026-03-08 [Kafka] Configurable max poll interval and records
2026-03-07 [DICOM] Fix null pixel data handling in dicom-converter

Additional Context

## Motivation

Power users working on multiple long-running projects benefit greatly from being
able to quickly identify and resume the right context without having to open each
session to inspect it.