refactor: two-section sidebar with agent detail panel, theme system with 9 built-in themes

sidebar layout:
- top section: workspace list with accent bar, git branch, state dots
- bottom section: agent detail panel sorted by urgency with braille spinners
- active workspace always highlighted (subtle bg in terminal mode, brighter in nav)
- palette-driven colors throughout (no hardcoded colors in ui.rs)

theme system:
- 9 built-in themes: catppuccin (default), tokyo-night, dracula, nord, gruvbox, one-dark, solarized, kanagawa, rose-pine
- config: theme.name selects base, theme.custom overrides individual tokens
- 15 color tokens: accent, surface0/1, overlay0/1, text, subtext0, mauve, green, yellow, red, blue, teal, peach
- legacy ui.accent still works for backward compatibility

infrastructure:
- Palette struct on AppState, ready for future theme system extensions
- git_branch() reads .git/HEAD for branch detection
- PaneDetail struct with urgency sorting for agent panel
- spinner_tick counter for braille animation (~8fps on 60fps render loop)

1 files changed, 11 insertions, 0 deletions

diff --git a/README.md b/README.md
index 2b7fe998..4fcdf859 100644
--- a/README.md
+++ b/README.md
@@ -154,6 +154,17 @@ print the full default config with:
 herdr --default-config
 ```
 
+### themes
+
+herdr ships with 9 built-in themes: catppuccin (default), tokyo night, dracula, nord, gruvbox, one dark, solarized, kanagawa, and rosé pine.
+
+```toml
+[theme]
+name = "tokyo-night"
+```
+
+you can also override individual color tokens on top of any base theme. see [`CONFIGURATION.md`](./CONFIGURATION.md) for the full token reference.
+
 for all keybindings, onboarding, notification, sound, UI options, and environment variables, see [`CONFIGURATION.md`](./CONFIGURATION.md).
 
 ## session persistence