style(tui): picker highlighted item → Aurora blue (v0.6.4)

Operator request: the agent picker's highlighted selection should
get the brand-color treatment — Aurora blue background — instead of
the v0.6.1 dark-30 muted bg.

## Two-fix landing

**The selector**: v0.6.1's `ListView > ListItem.--highlight` (double
dash) never actually matched. Textual's class is `-highlight` (single
dash). The v0.6.1 "fix" silently fell through to Textual's defaults,
which happened to be invisible because $block-cursor-background was
configured but the selector path didn't reach the rendered widget.

Probed live: `item.classes = frozenset({'-highlight'})`. Selector
corrected, plus dropped the `>` combinator since Textual's internal
DOM puts wrappers between `ListView` and `ListItem`.

**The background**: explicit `#agent-list:focus ListItem.-highlight
{ background: $primary }` — Aurora blue (#6388D8) for the focused-
list highlight band.

**The contrast**: bright-blue id-line text on Aurora-blue background
would be unreadable. Highlighted-state child overrides:

  .agent-id-line  →  $au-bright-white (#cce7ec) + bold
  .agent-desc     →  $au-bright-80    (#b3cbcf)

Non-highlighted items keep their default colors (bright-blue id +
bright-70 desc on App bg).

## Verified live

13 fill rects of `#6388d8` in the picker SVG export (was 0
before this commit). Other Australis brand colors intact:
chrome surface #373b46, dark-50 #6e7882, dark-30 #414751.

## Tests

241/241 GREEN; ruff clean. No test rewrites needed — picker tests
assert structure (widget tree, key bindings), not colors.

Patch bump (v0.6.3 → v0.6.4) — cosmetic; no public-API change.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.6.3"
+version = "0.6.4"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"