feat: navigate the tag autocomplete with the arrow keys (#177)

[matheuswhite]

Closes #177.

Problem

When the @tag autocomplete pop-up is open there is no way to move through the list — the only way to reach a tag is to type it until it becomes the first entry, which is tedious for long tag names or long lists.

Solution

While the pop-up is showing, the arrow keys drive it:

• ↑ / ↓ move a visible highlight through the list (overriding command-history navigation, which is restored the moment the pop-up is not showing).
• The highlighted entry is drawn as a full-width cyan bar so the current selection is unmistakable.
• Tab completes the highlighted entry instead of always the first one.

The list also windows around the selection so the highlight stays on screen, with a trailing ... when more entries exist below.

Implementation

• TagList gains a selected index with select_next/select_prev (clamped at both ends), selected(), get_selected_autocomplete() and has_suggestions(). The index resets to the top whenever the list is rebuilt (every keystroke) and on clear().
• inputs_task.rs: the Up/Down arms branch on has_suggestions() before falling through to history; handle_tab_input uses the selected entry.
• graphics_task.rs: draw_autocomplete_list windows the list around the selection and highlights the selected row.

Testing

• 6 new unit tests in infra/tags.rs for selection navigation, clamping, reset-on-rebuild and has_suggestions.
• New e2e test tag_autocomplete_down_arrow_then_tab_completes_selected_tag (tests/tui_e2e.rs): @ta → ↓ → Tab resolves to @tag2.
• Full suite green: 185 unit tests, 11 e2e (1 pre-existing macOS-only ignore). Verified manually in the real TUI (highlight, ↑/↓, clamp, window scroll, Tab of the selected item).

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