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.

persistent-memory.md

@@ -32,9 +32,9 @@ separate dev team rather than an in-tree Worldtree tool.
 
 ## Current state / in-flight
 
-_As of 2026-05-24 (post-v0.6.3 Australis restored, $background → black):_
+_As of 2026-05-24 (post-v0.6.4 picker Aurora-blue highlight):_
 
-**Status: v0.6.3 shipped.** Nine core issues complete (`sse_client`
+**Status: v0.6.4 shipped.** Nine core issues complete (`sse_client`
 #1, `sessions` #2, `cli` #3, `tui` #4, `--end-user-id` #5, TUI
 startup error visibility #6, presenter contract semantics amendment
 #12, startup agent picker #8, §5 layout reshape + Tools pane #13)
@@ -51,8 +51,9 @@ Static in the footer (static "Tools" v1; dynamic when more tabs
 land). CLI mode (--send) unaffected by design — INV-018.
 
 Last commits on `main`:
-- v0.6.3 style(tui): restore Australis palette, only $background → pure black
-- `d845b20` style(tui): neutralize Australis dark palette — bg no longer blue-tinted (v0.6.2, reverted)
+- v0.6.4 style(tui): picker highlighted item → Aurora blue (brand selection treatment)
+- `ac690c1` style(tui): restore Australis palette, only $background → pure black (v0.6.3)
+- `d845b20` style(tui): neutralize Australis dark palette (v0.6.2, reverted)
 - `8463eb2` style(tui): kill remaining blue + thinking-current into pane (v0.6.1)
 - `cfee89a` refactor(tui): streaming + turn headers + Thinking pane (v0.6.0)
 - `7106af5` style(tui): UI polish pass — terminal label colors, placeholders (v0.5.1)