feat: conversation browser with rename, search, and keyboard-driven navigation

Problem

As Claude Code usage grows, conversations accumulate fast. Currently, /conversations lists sessions by ID and date with no context, no rename, no search. It's like having a file system without a file manager — you navigate blind and remember paths by heart.

For users who live in the terminal (the exact audience Claude Code targets), this is a surprising gap. We're used to information-dense, keyboard-first interfaces — Far Commander, Total Commander, Midnight Commander, ranger — where browsing hundreds of items is instant.

Proposal

A conversation browser accessible via /conversations (or a new /browse) with:

| Feature | Description |
|---|---|
| Rename | Give sessions meaningful names ("IEvent XPO bug", "Distribution module setup") instead of UUIDs |
| Search | Full-text search across conversation history — find that session where you solved a specific problem |
| Preview | Show first message / summary on highlight (like panel preview in dual-pane managers) |
| Keyboard navigation | j/k or arrows to browse, Enter to resume, F2 rename, / to search — zero mouse required |
| Project grouping | Group by working directory / project automatically |

Why this matters

• Institutional memory: Memory files help, but they're summaries. Sometimes you need the actual conversation ("how exactly did we debug that?")
• Scale: Power users will have 100+ sessions per project within weeks. Without organization, /resume becomes a guessing game
• Terminal-native UX: Claude Code already uses a TUI. A keyboard-driven browser fits naturally — no new paradigm needed

Prior art

• Far Commander / Midnight Commander — dual panel, keyboard-first, information-dense
• fzf — fuzzy finder for terminal, minimal UI, maximum utility
• Zellij / tmux session management — named sessions, easy switching

Even an fzf-style fuzzy picker over conversations (with rename support) would be a massive improvement over the current flat list.