feat(v2): complete multi-LLM providers, TUI redesign, and advanced agent features

Multi-LLM Provider Support:
- Add llm-core crate with LlmProvider trait abstraction
- Implement Anthropic Claude API client with streaming
- Implement OpenAI API client with streaming
- Add token counting with SimpleTokenCounter and ClaudeTokenCounter
- Add retry logic with exponential backoff and jitter

Borderless TUI Redesign:
- Rewrite theme system with terminal capability detection (Full/Unicode256/Basic)
- Add provider tabs component with keybind switching [1]/[2]/[3]
- Implement vim-modal input (Normal/Insert/Visual/Command modes)
- Redesign chat panel with timestamps and streaming indicators
- Add multi-provider status bar with cost tracking
- Add Nerd Font icons with graceful ASCII fallbacks
- Add syntax highlighting (syntect) and markdown rendering (pulldown-cmark)

Advanced Agent Features:
- Add system prompt builder with configurable components
- Enhance subagent orchestration with parallel execution
- Add git integration module for safe command detection
- Add streaming tool results via channels
- Expand tool set: AskUserQuestion, TodoWrite, LS, MultiEdit, BashOutput, KillShell
- Add WebSearch with provider abstraction

Plugin System Enhancement:
- Add full agent definition parsing from YAML frontmatter
- Add skill system with progressive disclosure
- Wire plugin hooks into HookManager

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

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

crates/llm/openai/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "llm-openai"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+description = "OpenAI GPT API client for Owlen"
+
+[dependencies]
+llm-core = { path = "../core" }
+async-trait = "0.1"
+futures = "0.3"
+reqwest = { version = "0.12", features = ["json", "stream"] }
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+tokio = { version = "1", features = ["sync", "time", "io-util"] }
+tokio-stream = { version = "0.1", default-features = false, features = ["io-util"] }
+tokio-util = { version = "0.7", features = ["codec", "io"] }
+tracing = "0.1"