feat: add theme hot-reload functionality

[malhashemi]

Summary

Adds the ability to reload themes without restarting opencode. Provides multiple reload triggers to support different workflows:

Trigger	Use Case
/reload-theme command	Manual reload when needed
File watchers	Auto-reload on config/theme file edits
Window resize	Re-detect terminal colors for "system" theme
SIGUSR1/SIGUSR2	Integration point for external automation

Also makes "system" theme background transparent to support terminal opacity.

Changes

 theme.tsx        | +178  Core: reloadTheme(), file watchers, signal handlers
 app.tsx          | -66/+9  Moved getTerminalBackgroundColor, added command
 autocomplete.tsx | +5     Added /reload-theme slash command

Architecture

flowchart LR
    subgraph Triggers
        A[Command]
        B[File change]
        C[Resize]
        D[Signal]
    end

    A & B & C & D --> E[reloadTheme]
    E --> F[Re-query colors]
    F --> G[Update store]
    G --> H[UI re-renders]

Loading

Key Decisions

Why move getTerminalBackgroundColor to theme.tsx?
Needed by both initial detection (app.tsx) and reload (theme.tsx). Moving it eliminates duplication.

Why fs.watch() instead of @parcel/watcher?
Simpler API, no extra dependencies, sufficient for watching 2 files.

Why transparent background for "system" theme?
Allows terminal opacity/blur to show through while keeping panel/menu backgrounds solid.

Why SIGWINCH handler?
Event-driven color re-detection without polling. Only active for "system" theme.

Testing

# Command
/reload-theme

# File watcher
echo '{"theme":"dracula"}' > ~/.config/opencode/opencode.json

# Signal (Unix)
kill -USR1 $(pgrep opencode)

# Resize (system theme)
# Change terminal colors, then resize window

Checklist

• Commands work (palette + slash)
• File watchers trigger reload
• Signal handlers work (Unix)
• Proper cleanup on unmount
• TypeScript passes
• Follows AGENTS.md style

Fixes #815