feat(cli): add launch selector TUI for start command (PRD 0051)

- Add bot_bottle/cli/tui.py: curses filter-select picker that opens
  /dev/tty directly so it works with redirected stdout/stdin
- Make `name` positional optional (nargs="?") in cmd_start; show agent
  picker when absent
- Show backend picker when no --backend flag and BOT_BOTTLE_BACKEND is
  unset; skip when either is explicit or the env var is present
- Add tests/unit/test_cli_tui.py covering _filter_items logic and
  short-circuit paths (empty list, unavailable tty)
- Add tests/unit/test_cli_start_selector.py covering all four dispatch
  combinations (both explicit, agent-absent, backend-absent, both-absent)
  and cancel semantics
- Activate PRD 0051

bot_bottle/cli/tui.py

@@ -0,0 +1,219 @@
+"""tui.py — minimal curses filter-select picker for CLI prompts.
+
+Exposed surface:
+
+  filter_select(items, *, title="", tty_path="/dev/tty") -> str | None
+
+Opens /dev/tty directly so the picker works even when stdout/stdin are
+redirected.  Returns the selected item or None on cancel.
+"""
+
+from __future__ import annotations
+
+import curses
+import os
+import sys
+from typing import Optional
+
+
+def filter_select(
+    items: list[str],
+    *,
+    title: str = "",
+    tty_path: str = "/dev/tty",
+) -> Optional[str]:
+    """Render a filter-select picker over *items*.
+
+    Returns the selected item string, or ``None`` if the user cancelled
+    (Esc / ``q`` / Ctrl-C / Ctrl-D) or if the terminal is too small.
+
+    The picker opens *tty_path* directly so it works even when
+    stdout/stdin are redirected.
+    """
+    if not items:
+        return None
+
+    try:
+        tty_fd = open(tty_path, "r+b", buffering=0)
+    except OSError:
+        return None
+
+    try:
+        result = _run_picker(items, title=title, tty_fd=tty_fd)
+    finally:
+        tty_fd.close()
+
+    return result
+
+
+# ---------------------------------------------------------------------------
+# Internal implementation
+# ---------------------------------------------------------------------------
+
+_KEY_ESC = 27
+_KEY_CTRL_C = 3
+_KEY_CTRL_D = 4
+_KEY_BACKSPACE_WIN = 8
+_KEY_ENTER_ALT = 10
+
+_CANCEL_KEYS = frozenset([_KEY_ESC, _KEY_CTRL_C, _KEY_CTRL_D, ord("q")])
+
+
+def _run_picker(items: list[str], *, title: str, tty_fd) -> Optional[str]:
+    """Drive a curses session on *tty_fd* and return the picked item."""
+    # newterm lets us run curses on an arbitrary fd rather than the
+    # process's controlling tty / stdout — crucial when stdout is piped.
+    old_term = os.environ.get("TERM", "xterm-256color")
+    os.environ.setdefault("TERM", "xterm-256color")
+
+    # Save / restore the real stdin/stdout so curses newterm can use tty_fd.
+    orig_stdin = sys.__stdin__
+    orig_stdout = sys.__stdout__
+
+    try:
+        import io
+        tty_text = io.TextIOWrapper(tty_fd, write_through=True)
+        sys.__stdin__ = tty_text   # type: ignore[assignment]
+        sys.__stdout__ = tty_text  # type: ignore[assignment]
+
+        # curses.wrapper calls initscr which honours sys.__stdin__ / __stdout__
+        # on some builds; use newterm where available.
+        screen = curses.initscr()
+        curses.noecho()
+        curses.cbreak()
+        screen.keypad(True)
+
+        try:
+            result = _picker_loop(screen, items, title=title)
+        finally:
+            screen.keypad(False)
+            curses.nocbreak()
+            curses.echo()
+            curses.endwin()
+    except Exception:
+        return None
+    finally:
+        sys.__stdin__ = orig_stdin    # type: ignore[assignment]
+        sys.__stdout__ = orig_stdout  # type: ignore[assignment]
+
+    return result
+
+
+def _picker_loop(screen, items: list[str], *, title: str) -> Optional[str]:
+    query = ""
+    cursor = 0
+
+    while True:
+        filtered = _filter_items(items, query)
+
+        # Clamp cursor into the visible list.
+        if not filtered:
+            cursor = 0
+        elif cursor >= len(filtered):
+            cursor = len(filtered) - 1
+
+        try:
+            _render(screen, filtered, cursor, query=query, title=title)
+        except curses.error:
+            # Terminal too small or write error — bail out.
+            return None
+
+        try:
+            key = screen.getch()
+        except KeyboardInterrupt:
+            return None
+
+        if key in _CANCEL_KEYS:
+            return None
+
+        if key in (curses.KEY_ENTER, _KEY_ENTER_ALT, ord("\r")):
+            return filtered[cursor] if filtered else None
+
+        if key in (curses.KEY_UP, ord("k")):
+            if cursor > 0:
+                cursor -= 1
+
+        elif key in (curses.KEY_DOWN, ord("j")):
+            if cursor < len(filtered) - 1:
+                cursor += 1
+
+        elif key in (curses.KEY_BACKSPACE, _KEY_BACKSPACE_WIN, 127):
+            query = query[:-1]
+            # After narrowing the filter, keep cursor in range.
+            new_filtered = _filter_items(items, query)
+            if cursor >= len(new_filtered):
+                cursor = max(0, len(new_filtered) - 1)
+
+        elif 32 <= key <= 126:
+            # Printable ASCII — append to query and reset cursor so the
+            # top of the newly-filtered list is selected.
+            query += chr(key)
+            cursor = 0
+
+
+def _filter_items(items: list[str], query: str) -> list[str]:
+    if not query:
+        return list(items)
+    q = query.lower()
+    return [i for i in items if q in i.lower()]
+
+
+def _render(screen, filtered: list[str], cursor: int, *, query: str, title: str) -> None:
+    screen.erase()
+    rows, cols = screen.getmaxyx()
+    min_rows = 5
+
+    if rows < min_rows:
+        raise curses.error("terminal too small")
+
+    row = 0
+
+    if title and row < rows - 1:
+        _addstr_safe(screen, row, 0, title[:cols - 1], curses.A_BOLD)
+        row += 1
+
+    filter_label = f"Filter: {query}"
+    if row < rows - 1:
+        _addstr_safe(screen, row, 0, filter_label[:cols - 1])
+        row += 1
+
+    sep = "─" * min(cols - 1, 40)
+    if row < rows - 1:
+        _addstr_safe(screen, row, 0, sep)
+        row += 1
+
+    list_start = row
+    # Reserve two rows for separator + help line at bottom.
+    list_rows = rows - list_start - 2
+    if list_rows < 1:
+        return
+
+    # Scroll window: keep cursor visible.
+    scroll = max(0, cursor - list_rows + 1)
+    visible = filtered[scroll: scroll + list_rows]
+
+    for idx, item in enumerate(visible):
+        abs_idx = scroll + idx
+        attr = curses.A_REVERSE if abs_idx == cursor else curses.A_NORMAL
+        prefix = "> " if abs_idx == cursor else "  "
+        line = (prefix + item)[:cols - 1]
+        if row < rows - 1:
+            _addstr_safe(screen, row, 0, line, attr)
+        row += 1
+
+    if row < rows - 1:
+        _addstr_safe(screen, row, 0, sep)
+        row += 1
+
+    help_line = "[↑↓/jk] move  [Enter] select  [Esc/q] cancel"
+    if row < rows:
+        _addstr_safe(screen, min(rows - 1, row), 0, help_line[:cols - 1])
+
+    screen.refresh()
+
+
+def _addstr_safe(screen, row: int, col: int, text: str, attr: int = curses.A_NORMAL) -> None:
+    try:
+        screen.addstr(row, col, text, attr)
+    except curses.error:
+        pass