Release v2.0.0

- **Help overlay (TUI)** — `F1` opens a centered modal listing all keybindings in grouped sections (Navigation, Editing, Overlays, AI/Shell). Styled identically to the picker overlay. Dismiss with `Esc` or `F1`.
- **`/help` slash command (REPL)** — prints the same keybinding groups with ANSI styling in `--no-tui` mode.
- **Reverse search (TUI)** — `Ctrl+R` opens bash/zsh-style reverse incremental search in the input bar. Type to filter history, `Ctrl+R` cycles matches, `Enter` accepts, `Esc` cancels.
- **Session browser overlay (TUI)** — `Ctrl+S` opens a centered overlay listing all sessions with title, age, and message count. Active session marked with `●`. Navigate with `↑/↓`, `Enter` to resume, `d` to delete (with confirmation), `r` to rename (inline editing), `n` for new session, `Esc` to close.
- **`/sessions` and `/session` slash commands (REPL)** — `/sessions` lists sessions; `/session resume <N>` switches to a session by number; `/session delete <N>` deletes with confirmation; `/session rename <N> <title>` renames.
- **`SessionStore::delete()` and `SessionStore::rename()`** — new methods for session management (used by both TUI and REPL).
- **Runtime profile switching in REPL and TUI** — `/profile [use <name>]` switches the active profile at runtime without editing `config.toml`; `/profile [list]` lists all configured profiles with the active one marked (`●`). Works in both the rustyline REPL and the TUI input bar.
- **Account/Profile/Model picker overlay (TUI)** — `Ctrl+P` opens a centered three-tab popup:
  - **Account** tab: switch the active account per provider for the current session
  - **Profile** tab: switch the active profile (all four roles change at once)
  - **Model** tab: pick a specific model for the current session (session-scoped override, no config change)
  - `[Tab]` cycles tabs, `[↑↓]` navigates, `[Enter]` applies, `[Esc]` closes. Active selection marked with `●`.
- **Session-scoped model override** — picking a model in the overlay (or typing `/profile use <name>` to switch profiles) updates the status bar immediately without restarting.
- **`AiContext::switch_account()`** — rebuilds the provider registry when the active account changes, re-routing credentials to the newly selected account.

- `ait profile use <name>` (CLI subcommand) previously only validated the name and printed a hint. The REPL now has its own `/profile use <name>` slash command for actual runtime switching.