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>

crates/core/agent/Cargo.toml

@@ -13,6 +13,7 @@ tokio = { version = "1", features = ["full"] }
 futures-util = "0.3"
 tracing = "0.1"
 async-trait = "0.1"
+chrono = "0.4"
 
 # Internal dependencies
 llm-core = { path = "../../llm/core" }
@@ -22,6 +23,7 @@ tools-bash = { path = "../../tools/bash" }
 tools-ask = { path = "../../tools/ask" }
 tools-todo = { path = "../../tools/todo" }
 tools-web = { path = "../../tools/web" }
+tools-plan = { path = "../../tools/plan" }
 
 [dev-dependencies]
 tempfile = "3.13"