feat(fleet-tui-poc): Tab-triggered ⌘K section-jump overlay (#15)

Adds the iOS-style "Jump to section" grid the user mocked: pressing
prefix Tab inside a codex-fleet tmux session pops up a 3×2 card grid
(Overview · Fleet · Plan · Waves · Review), and number keys 1–5 dispatch
`tmux select-window -t <session>:<window>` then exit. Esc / 0 / q / Tab
dismiss. Enter activates the card matching --active <window>, falling
back to section 1.

  - rust/fleet-tui-poc/src/main.rs:
    new Overlay::SectionJump variant; SECTIONS metadata table;
    render_section_jump + render_jump_card with the iOS card chrome
    (icon badge + key badge + title + sub + footer line, active card
    in IOS_TINT); App::section_active threads the caller's "current
    window" hint into the renderer; run() gains session + active_section
    params and a new key branch that handles 1–5 / Esc / Enter / Tab;
    main() learns --session, --active flags and a CODEX_FLEET_TMUX_SESSION
    env fallback. parse_overlay accepts "section-jump" / "jump".

  - scripts/codex-fleet/bin/section-jump-chooser.sh (new):
    thin launcher mirroring pane-context-menu-chooser.sh — resolves
    the workspace target dir (release → debug → loud failure), infers
    session + active window via `tmux display-message`, exec's the
    fleet-tui-poc binary in single-shot mode.

  - scripts/codex-fleet/tmux-bindings.conf:
    binds `prefix Tab` to display-popup invoking the chooser. Bare Tab
    is intentionally left untouched so shell tab-completion inside
    panes is unaffected.

Smoke-tested by running the binary in a detached tmux session with
`--overlay section-jump --session codex-fleet --active fleet`:
overlay renders with the expected card grid + footer hints, Esc exits
cleanly (session vanishes), and `cargo build -p fleet-tui-poc` is
clean (only the pre-existing ctx_menu_items dead-code warning from the
in-flight iOS context-menu lane).

Co-authored-by: NagyVikt <nagy.viktordp@gmail.com>

Author: NagyVikt