feat(coding-agent): add session naming via /name command and extension API

- Add SessionInfoEntry type for session metadata
- Add /name <name> command to set session display name
- Add pi.setSessionName() and pi.getSessionName() extension API
- Session selector shows name (in warning color) instead of first message when set
- Session name included in fuzzy search
- /session command displays name when set

closes #650

packages/coding-agent/README.md

@@ -240,6 +240,7 @@ The agent reads, writes, and edits files, and executes commands via bash.
 | `/export [file]` | Export session to self-contained HTML |
 | `/share` | Upload session as secret GitHub gist, get shareable URL (requires `gh` CLI) |
 | `/session` | Show session info: path, message counts, token usage, cost |
+| `/name <name>` | Set session display name (shown in session selector) |
 | `/hotkeys` | Show all keyboard shortcuts |
 | `/changelog` | Display full version history |
 | `/tree` | Navigate session tree in-place (search, filter, label entries) |