style(tui): UI polish pass (v0.5.1)

Cosmetic refinements on top of v0.5.0's content-only main pane. No
behavior change; ships as a patch bump.

## Color signal — terminal labels tinted per outcome

The transcript's [done]/[error]/[cancelled] labels were plain
foreground (Australis #a9bcc3 white), which made them slow to scan
against the surrounding assistant text. Now tinted per outcome:

- [done]      → Aurora green   (#16B866 / $success)
- [error]     → Dawn red       (#ff491a / $error)
- [cancelled] → Dawn yellow    (#e1c631 / $warning)

The post-Done Rule() separator is also tinted to Australis dark-60
(#86929d) so the streamed-text → markdown-body boundary reads as
chrome, not a content artifact.

## Empty-state placeholders

Tools and Debug panes were stark-empty before any turn fired — easy
to misread as "the pane is broken." Now show placeholder lines on
mount in Sea dark-50 italic:

  Tools tab: (no tool events yet — start a turn that uses tools)
  Debug tab: (waiting for telemetry — start a turn)

The placeholders scroll off naturally as real events fill the panes.

## Live thinking widget self-explains

The thinking-current Static at the top of the right column used to
just display raw thinking content with no context — an operator
glancing at the screen mid-stream might not realize they were
looking at LLM chain-of-thought. Now prefixed with "thinking… " so
the widget self-identifies.

## Spacing + chrome

- Transcript / tools-log / debug-log: 1-cell horizontal padding so
  content doesn't hug the column border.
- thinking-current: italic text-style on top of the dark-60 color,
  so the live-preview band is visually distinct from solid-colored
  log content.
- Active tab in TabbedContent: Aurora bright-cyan label + bold
  text-style, so the eye lands on the currently selected pane.
- Input placeholder text: tinted to Sea dark-50 so it reads as
  placeholder, not content.

## Test impact

3 new tests added (test_done_label_styled_success,
test_empty_state_placeholders_present, plus the polish hits
test_thinking_widget_truncation / test_thinking_coalesce updated for
the "thinking… " prefix). 4 existing tests that checked
`isinstance(w, str) and w.startswith("[done]")` updated to use the
_text_of helper (terminal labels are now RichText, not str).
_spy_writes helper widened to accept positional args after Textual's
internal deferred-render path started passing them positionally
post-Resize.

241/241 GREEN; ruff clean. Live smoke against personal Worldtree
confirmed: Done line renders in Aurora green #16B866 verbatim;
both placeholder lines appear in dark-50; thinking widget shows
"thinking… <content>" during a turn.

Patch bump (v0.5.0 → v0.5.1) per SemVer etiquette: purely cosmetic;
no signature change; no caller-visible behavioral shift.

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-v0.5.0 content-only main pane + Debug tab):_
+_As of 2026-05-24 (post-v0.5.1 UI polish pass):_
 
-**Status: v0.5.0 shipped.** Nine core issues complete (`sse_client`
+**Status: v0.5.1 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)
@@ -51,7 +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`:
-- v0.5.0 refactor(tui): content-only main pane + Debug tab + chrome dark
+- v0.5.1 style(tui): polish pass — colored terminal labels, placeholders, padding
+- `ffd22fb` refactor(tui): content-only main pane + Debug tab + chrome dark (v0.5.0)
 - `2756f5f` style(tui): apply Australis theme to TUI chrome + widgets (v0.4.1)
 - `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)