restore_autosave_interactively: magic 1-5/6 pagination keys, digit-named sessions unselectable, duplicates pagination.py and autosave_menu

Problem

The interactive autosave restore prompt in code_puppy/session_storage.py:218-292 (restore_autosave_interactively) hard-codes its pagination contract in a way that breaks for the user:

PAGE_SIZE = 5
...
# Numeric choice: 1-5 select within current page; 6 advances page
if selection.isdigit():
    num = int(selection)
    if num == 6 and total > PAGE_SIZE:
        page = (page + 1) % ((total + PAGE_SIZE - 1) // PAGE_SIZE)
        continue
    if 1 <= num <= 5:
        ...

Issues:

1. Magic numbers 5 and 6 are sprinkled across rendering (render_page, line 246: emit_system_message(f" [6] {label}")), parsing (lines 275-285), and the prompt string ("Pick 1-5 ... 6 for next"). Changing PAGE_SIZE to anything else silently breaks the "6 = next page" convention — the option key should be derived (PAGE_SIZE + 1) or better, a letter (n for next).
2. Sessions whose name is a digit 1-6 cannot be selected by name. The selection.isdigit() branch consumes the input before the exact-name match below it, and a session literally named 6 triggers page-flipping instead. ("In the face of ambiguity, refuse the temptation to guess.")
3. This menu duplicates the nicer pagination already extracted into command_line/pagination.py (get_page_bounds, get_total_pages, etc.) and the full-blown TUI in command_line/autosave_menu.py — three implementations of "browse autosaves" now exist (this text prompt, autosave_menu.py, and the /autosave_load marker flow).

Suggested fix

• Derive the next-page key: next_key = str(PAGE_SIZE + 1) and interpolate it in both rendering and parsing, or switch to n/p keys.
• Use command_line/pagination.py helpers for page math.
• Longer term: route the startup restore through autosave_menu.py's picker (with a plain-text fallback when not a TTY) so there is one obvious implementation.

Filed by Zen Reviewer C (code-puppy-60635a)

Start in code_puppy/session_storage.py at restore_autosave_interactively, then read command_line/pagination.py and command_line/autosave_menu.py to compare the existing pagination approaches. Done should include pagination keys derived from the page size, digit-named sessions remaining selectable, and consistent page navigation without introducing another autosave-browsing implementation.