v0.5.0 — UX polish + TUI rendering fix

Highlights

Critical fix — TUI bodies no longer render empty

All 8 screens previously used height: NN% CSS, which Textual 8.2.5
collapses to 0 inside a TabPane. Every tab opened to an empty viewport.
Switched the screens and TabbedContent to 1fr fractional units;
DataTable and side panes now share remaining space correctly.

UX polish

• Per-screen header on every tab — a single Rich-markup line stating
  what the tab shows and the key actions available.
• Empty-state CTAs on Schedule (lists the 5 built-in templates),
  Audit (shows the cc-janitor stats snapshot recipe), and Dream
  (explains the safety-net concept).
• F1 tab-specific help modal — context-aware help for each tab.
• First-run Welcome modal with the 8-tab tour. Marker at
  ~/.cc-janitor/state/seen-welcome makes it one-shot; re-display via
  cc-janitor --tutorial.
• Filter-by-scope labels next to the source-filter dropdowns on
  Permissions / Memory / Hooks.
• Inline column legends on Sessions (Msgs/Size) and Permissions
  (Used90d, STALE/DUP).
• Screen-level BINDINGS on every screen so Footer surfaces the
  per-tab action keys.

Numbers

• 302 passing tests (was 290; +12 in tests/tui/test_ux_polish.py)
• All 4 CI jobs green (Ubuntu/Windows × Python 3.11/3.12)

Install / upgrade

pip install --upgrade cc-janitor
cc-janitor --tutorial   # see the new Welcome tour

See CHANGELOG.md for the full diff.