style(tui): neutralize Australis dark palette (v0.6.2)

Operator-flagged third pass: "overall background for the whole app is
blue." The previous "zero blue rects" investigations missed the
structural cause — Australis's design principle "all colors are
cooler than neutral" bakes a blue cast into every dark surface:

  Ice black    #222531 = RGB(34, 37, 49)   — blue +44% over red
  Sea bright   #373b46 = RGB(55, 59, 70)   — blue +27% over red
  Sea dark-30  #414751 = RGB(65, 71, 81)   — blue +25% over red
  Sea dark-60  #86929d = RGB(134,146,157)  — blue +17% over red

Every chrome surface inherits the lean. The user reading "the whole
app is blue" is correct — the SVG export just rendered hex values
that aren't named "blue" but ARE measurably blue-tinted.

## Fix: keep accents, neutralize darks

Australis brand signature lives in the ACCENTS — Aurora blue, cyan,
green; Dawn red, yellow. Those are unchanged. The Ice/Sea dark
palette is replaced with LAB-matched neutral grays (R=G=B) so the
chrome reads truly neutral:

  $background  #222531 → #1a1a1a   (neutral near-black)
  $surface     #373b46 → #2a2a2a   (neutral dark gray)
  $panel       #414751 → #3a3a3a   (neutral mid gray)
  $foreground  #a9bcc3 → #bdbdbd   (neutral light gray)
  $au-dark-30  #414751 → #3a3a3a
  $au-dark-40  #565f69 → #4f4f4f
  $au-dark-50  #6e7882 → #6b6b6b
  $au-dark-60  #86929d → #878787
  $au-bright-70 #9daeb6 → #9e9e9e
  $au-bright-80 #b3cbcf → #bdbdbd
  $au-bright-white #cce7ec → #e0e0e0

`_AU_DEMOTED` and `_AU_DEMOTED_FAINT` constants (Rich Text styling
for demoted telemetry + placeholders) updated to the neutral
equivalents. The Aurora bright variants (`$au-bright-blue`,
`$au-bright-cyan`, `$au-bright-green`) stay verbatim — those are
where the brand voice lives.

## What this preserves vs sacrifices

**Preserved**:
- Aurora accents: focus borders, active-tab indicator, pane-name
  widget, user-prompt echo all still render in cyan/blue/green.
- Done/Error/Cancelled labels still tinted in Aurora green / Dawn
  red / Dawn yellow.
- Identity widget still Aurora bright-blue.
- The "Australis" theme name + variable slugs ($au-*) — downstream
  CSS rules don't have to change.

**Sacrificed**:
- The "all colors cooler than neutral" Australis design principle.
  Deliberate per-operator-feedback deviation; documented in the
  AUSTRALIS_THEME docstring as a v0.6.2 conscious break with spec.

## Tests + smoke

241/241 GREEN; ruff clean. Live screenshot exports:
- Main app: chrome colors are #1a1a1a / #2a2a2a / #3a3a3a / #6b6b6b
  / #bdbdbd — all neutral grays. Aurora accents preserved as
  textual highlights.
- Agent picker: same — neutral chrome, Aurora accents intact for
  highlighted item border + agent-id-line.

Patch bump (v0.6.1 → v0.6.2): purely cosmetic palette adjustment;
no public-API change.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.6.1"
+version = "0.6.2"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"