feat(cli): richer "?" help panel with current-session context

[emal-avala]

Summary

The existing ? help panel was hand-written and showed a curated subset of slash commands. Every new /command required a matching edit in repl.rs, and we routinely forgot (/brief, /rules, /reload, /ctxviz, /editor, /output-style, /copy, /tag, and /keybindings were all missing from the panel).

What changed

1. Extracted into render_help_panel(&AppState) so it can evolve in one place.
2. New "Current session" block at the top with live values:
   • model
   • permission mode
   • active modifier badges ([plan, brief, +N dirs])
   • context usage (tokens, % of window, cost) and turn count
3. Commands list is now auto-generated from the COMMANDS array — every non-hidden entry shows up, sorted alphabetically, with description and aliases. New commands land with no help-panel edits needed.
4. Keyboard/input section unchanged in content but column-aligned so entries don't drift when widths differ.

Example output

  Current session
  ────────────────────────────────────────────────────────────
  model         your-preferred-model  [plan, brief]
  permissions   ask
  usage         18243 tokens · 9% of 200000 · $0.0421 (turn 6)

  Keyboard & input
  ────────────────────────────────────────────────────────────
  ! command          run shell command directly
  @ path/file        inline file contents in the prompt
  & prompt           run prompt in background
  /name              slash command — Tab to complete
  \ + Enter          continue on next line
  Tab                auto-complete slash commands
  Ctrl+R             search prompt history
  Esc / Ctrl+C       cancel (Ctrl+C twice to exit)
  Ctrl+D             exit REPL

  Commands (53)
  ────────────────────────────────────────────────────────────
  /add-dir             Add a directory to the working set
  /agents              List available agent types
  /autofix-pr          Check out a PR, fix failures, push back
  /brief               Toggle brief mode (terse responses, ≤3 sentences)
  /copy                Copy the last assistant message to the system clipboard
  /ctxviz              Per-category token breakdown of the current context  (alias: /context-viz)
  /editor              Compose a multi-line prompt in \$EDITOR and submit it  (alias: /ed)
  /install-github-app  Walk through \`gh\` CLI setup ...  (alias: /gh-setup)
  ... (all COMMANDS, auto-generated)

Test plan

• cargo fmt --all — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo test -p agent-code --test smoke — 4/4 pass
• Manual: type ? in REPL → panel renders with live session state and all commands