feat(ui): add autocomplete, command help, and streaming improvements

TUI Enhancements:
- Add autocomplete dropdown with fuzzy filtering for slash commands
- Fix autocomplete: Tab confirms selection, Enter submits message
- Add command help overlay with scroll support (j/k, arrows, Page Up/Down)
- Brighten Tokyo Night theme colors for better readability
- Add todo panel component for task display
- Add rich command output formatting (tables, trees, lists)

Streaming Fixes:
- Refactor to non-blocking background streaming with channel events
- Add StreamStart/StreamEnd/StreamError events
- Fix LlmChunk to append instead of creating new messages
- Display user message immediately before LLM call

New Components:
- completions.rs: Command completion engine with fuzzy matching
- autocomplete.rs: Inline autocomplete dropdown
- command_help.rs: Modal help overlay with scrolling
- todo_panel.rs: Todo list display panel
- output.rs: Rich formatted output (tables, trees, code blocks)
- commands.rs: Built-in command implementations

Planning Mode Groundwork:
- Add EnterPlanMode/ExitPlanMode tools scaffolding
- Add Skill tool for plugin skill invocation
- Extend permissions with planning mode support
- Add compact.rs stub for context compaction

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -15,6 +15,8 @@ members = [
     "crates/tools/bash",
     "crates/tools/fs",
     "crates/tools/notebook",
+    "crates/tools/plan",
+    "crates/tools/skill",
     "crates/tools/slash",
     "crates/tools/task",
     "crates/tools/todo",