feat: wire autocomplete state and input handling

[yamaceay]

Context

PR 4 of 5 — slash-command autocomplete series.

Dependency / merge order:

#136 → #131 → #132 → #133 → #134 (this PR) → #135

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Why

With the command registry (PR #131) and current_line() (PR #133) in place,
this PR adds the data-flow layer: detecting when the user types a / prefix,
tracking which commands match, maintaining the selection index, and handling
the keyboard shortcuts that control the popup. No rendering yet — that's
PR #135.

Separating state/wiring from rendering keeps each PR reviewable on its own and
makes the renderer easier to test in isolation.

What

state.rs — autocomplete state in AppState

Three new fields:

Field	Type	Purpose
autocomplete_active	bool	Whether the popup is currently shown
autocomplete_query	String	Text after / on the current line
autocomplete_selected	usize	Index of the highlighted row

Three new helper methods:

• open_autocomplete(query) — opens/refreshes the popup; resets selection to 0.
• close_autocomplete() — hides the popup and clears all state.
• move_autocomplete_selection(delta, count) — moves selection by ±1,
  wrapping with rem_euclid so arrow keys cycle naturally.

input.rs — input layer changes

• InputManager.autocomplete_active: bool — mirrors AppState so key
  handling can intercept Up/Down/Tab/Escape without locking AppState.
• slash_prefix() -> Option<String> — reads current_line(), returns
  Some(text_after_slash) or None. Called after every keystroke.
• Three new KeyEventResult variants:

Variant	When emitted
UpdateAutocomplete { query: Option<String> }	After every keystroke / paste — None = close, Some(q) = open/refresh
MoveAutocomplete(i32)	Up (-1) / Down (+1) while popup is open
SelectAutocomplete	Tab while popup is open

• Escape while autocomplete_active emits UpdateAutocomplete { query: None }
  (closes popup) instead of propagating as a session-cancel event.
• Existing tests updated for the new return type of the _ => arm.
• New tests: slash detection, arrow interception, Tab, Escape.

app.rs — event loop wiring

Three new match arms:

• UpdateAutocomplete — filters all_commands() by prefix match on name
  or aliases; calls state.open_autocomplete / state.close_autocomplete;
  syncs input_manager.autocomplete_active.
• MoveAutocomplete — delegates to state.move_autocomplete_selection.
• SelectAutocomplete — finds the selected command name, replaces
  "/<query>" on the current input line with "/<name> " using
  TextArea::replace_range, closes the popup.
• Bracketed-paste events also call slash_prefix() to keep state in sync.

Scope

state.rs, input.rs, app.rs.

Test plan

• cargo test — new unit tests pass:
  • test_slash_prefix_detected
  • test_slash_prefix_absent_for_normal_text
  • test_escape_closes_autocomplete_when_active
  • test_arrow_keys_navigate_autocomplete_when_active
  • test_tab_selects_autocomplete_when_active
• cargo build --no-default-features passes.
• Manual (with PR feat: render slash-command autocomplete popup in TUI #135 on top): type /m → only /model matches;
  Tab → textarea shows /model .