TUI: show agf v<version> on the footer's far left #40

Proposal

Show the running agf version on the far left of the footer in the main TUI screens (Browse, GroupedBrowse, Preview, Help, etc.), e.g.:

agf v0.11.0  · ↑↓ nav · Tab agent · [/] summary · → detail · Enter select · ^S sort · ^G group · ^D delete · ? help · Esc quit

Why

• After upgrading (cargo install --force / brew upgrade subinium/tap/agf), users currently have to run agf --version separately to confirm which version they're actually on. With the binary pulled from a wrapper shell function and three possible install paths (cargo, Homebrew, prebuilt), this comes up more than expected.
• It's a useful "I just shipped this" sanity check for maintainers while dogfooding — exactly the situation that surfaced the v0.11.0 cache-stale bug (#37) where the binary said one thing and the cache said another. A visible version pinned to the rendered TUI makes "is this the new build or the old build?" a glance, not a side-quest.
• Cheap real estate. The footer's leftmost cell is currently empty padding before the help line.

Sketch

Cargo.toml already exposes the version as env!("CARGO_PKG_VERSION") at compile time, so no runtime cost:

const VERSION_LABEL: &str = concat!("agf v", env!("CARGO_PKG_VERSION"));

// in the footer row builder (e.g. inside ui_browse / ui_grouped_browse / etc.):
let _ = ui.container().pl(1).row(|ui| {
    ui.text(VERSION_LABEL).fg(GRAY_500);
    ui.text(" · ").fg(SEPARATOR);
    let _ = ui.help_colored(&[
        ("↑↓", "nav"), ("Tab", "agent"), ("[/]", "summary"),
        ("→", "detail"), ("Enter", "select"),
        ("^S", "sort"), ("^G", "group"), ("^D", "delete"),
        ("?", "help"), ("Esc", "quit"),
    ], GRAY_500, SEPARATOR);
});

Same treatment in the other top-level mode footers. For sub-mode footers (DeleteConfirm, ResumeSelect, etc.) it's optional — those screens are short-lived and the version is already visible on the screen they came from.

Style notes

• Color: dim (e.g. GRAY_500, same as the help text), not bold — informational, not a CTA.
• Format: agf v0.11.0 (one space after the program name, lowercase v, dot before separator). Keeps it readable next to the keybinding spans.
• Width: ~12 chars. Doesn't fight the help line for room on any terminal that already fits the existing footer.

Out of scope

• Showing the build profile (release/debug), git sha, etc. — clutter for a normal CLI user.
• Making it clickable / linking to GitHub release. Not a thing in this TUI lib.

Severity

Feature request, low priority. Pure quality-of-life.