style(tui): apply Australis theme to TUI chrome + widgets (v0.4.1)

Retheme the Textual TUI under the Australis Dark colorscheme
(github.com/lkraven/australis) — Aurora blue/cyan/green primary,
Ice neutrals (#222531 bg, #a9bcc3 fg, #cce7ec highlight), Sea
darks for chrome separators, Dawn accents reserved for terminal-
event labels (red/yellow). 16-color cool-tone palette with medium
contrast.

Mechanism: Textual `Theme` API. AUSTRALIS_THEME defined at module
scope mapped to semantic tokens (primary/secondary/accent/success/
warning/error/foreground/background/surface/panel) plus a Sea
variables block (`au-dark-30..60`, `au-bright-70/80/white`, plus
Aurora bright variants). Both `RatatoskrApp` and `AgentPickerApp`
register the theme in `__init__` and set `self.theme = "australis"`.

Per-widget styling via DEFAULT_CSS theme variables (no hex
sprinkled in CSS):

- `#identity` → `$au-bright-blue` (Aurora bright-blue).
- `#pane-name` → `$au-bright-cyan` (current-pane indicator).
- `#hint` → `$au-dark-60` (subtle Ctrl-C state line).
- `#prompt` border → `$panel` unfocused / `$primary` focused
  (focus highlight in Aurora blue).
- `#left-column` gets a `border-right: solid $panel` separator.
- `#thinking-current` color → `$au-dark-60` (matches demoted style).
- `#transcript` + `#tools-log` background → `$background`.
- Agent picker's highlighted ListItem → `$primary` bg +
  `$au-bright-white` fg.

Rich Text styling (where Textual's theme system doesn't apply):

- `_dim()` helper for demoted telemetry now uses explicit
  `#86929d` (Sea dark-60) instead of the terminal-dim filter
  `"dim"`. Renders consistently across emulators and stays
  anchored to the brand palette.
- User-prompt echo (`❯ <content>` in transcript) wrapped in
  `RichText` styled with `#42dcd1` (Aurora bright-cyan) — calls
  out the operator's voice in the primary palette.

CLI mode (`--send`) is unaffected by design — raw stdout has no
theme concept. The retheme is TUI-only.

236/236 tests GREEN; ruff clean. Live smoke against personal
Worldtree: `app.theme == "australis"`, all widget colors resolve
to expected Australis hex values (identity → `#a4c4ff`,
pane-name → `#42dcd1`, hint → `#86929d`).

One existing test adjusted: `test_worker_phase_demoted` previously
asserted `style == "dim"`; now asserts non-empty style (the demoted
style is now an explicit Australis hex, not the terminal "dim"
sentinel). Behavior-equivalent — the demotion intent is preserved.

Patch bump (v0.4.0 → v0.4.1) per SemVer etiquette: cosmetic
refinement of just-shipped surface, no public-API change.

persistent-memory.md

@@ -32,10 +32,9 @@ separate dev team rather than an in-tree Worldtree tool.
 
 ## Current state / in-flight
 
-_As of 2026-05-24 (post-v0.4.0 §5 entry point: layout reshape +
-Tools pane):_
+_As of 2026-05-24 (post-v0.4.1 Australis theme retheme):_
 
-**Status: v0.4.0 shipped.** Nine core issues complete (`sse_client`
+**Status: v0.4.1 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)
@@ -52,7 +51,8 @@ Static in the footer (static "Tools" v1; dynamic when more tabs
 land). CLI mode (--send) unaffected by design — INV-018.
 
 Last commits on `main`:
-- v0.4.0 feat(tui): issue #13 — §5 layout reshape + Tools pane
+- v0.4.1 style(tui): apply Australis theme to TUI chrome + widgets
+- `24e4371` feat(tui): issue #13 — §5 layout reshape + Tools pane (v0.4.0)
 - `d30be12` feat(sessions,cli,tui): issue #8 — startup agent picker (v0.3.0)
 - `c85f6bd` fix(tui): anchor layout via dock so Input never moves (v0.2.1)
 - `3b9c610` feat(cli,tui): issue #12 — presenter contract semantics amendment (v0.2.0)