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/core/agent/examples/git_demo.rs

@@ -0,0 +1,74 @@
+//! Example demonstrating the git integration module
+//!
+//! Run with: cargo run -p agent-core --example git_demo
+
+use agent_core::{detect_git_state, format_git_status, is_safe_git_command, is_destructive_git_command};
+use std::env;
+
+fn main() -> color_eyre::Result<()> {
+    color_eyre::install()?;
+
+    // Get current working directory
+    let cwd = env::current_dir()?;
+    println!("Detecting git state in: {}\n", cwd.display());
+
+    // Detect git state
+    let state = detect_git_state(&cwd)?;
+
+    // Display formatted status
+    println!("{}\n", format_git_status(&state));
+
+    // Show detailed file status if there are changes
+    if !state.status.is_empty() {
+        println!("Detailed file status:");
+        for status in &state.status {
+            match status {
+                agent_core::GitFileStatus::Modified { path } => {
+                    println!("  M {}", path);
+                }
+                agent_core::GitFileStatus::Added { path } => {
+                    println!("  A {}", path);
+                }
+                agent_core::GitFileStatus::Deleted { path } => {
+                    println!("  D {}", path);
+                }
+                agent_core::GitFileStatus::Renamed { from, to } => {
+                    println!("  R {} -> {}", from, to);
+                }
+                agent_core::GitFileStatus::Untracked { path } => {
+                    println!("  ? {}", path);
+                }
+            }
+        }
+        println!();
+    }
+
+    // Test command safety checking
+    println!("Command safety checks:");
+    let test_commands = vec![
+        "git status",
+        "git log --oneline",
+        "git diff HEAD",
+        "git commit -m 'test'",
+        "git push --force origin main",
+        "git reset --hard HEAD~1",
+        "git rebase main",
+        "git branch -D feature",
+    ];
+
+    for cmd in test_commands {
+        let is_safe = is_safe_git_command(cmd);
+        let (is_destructive, warning) = is_destructive_git_command(cmd);
+
+        print!("  {} - ", cmd);
+        if is_safe {
+            println!("SAFE (read-only)");
+        } else if is_destructive {
+            println!("DESTRUCTIVE: {}", warning);
+        } else {
+            println!("UNSAFE (modifies state)");
+        }
+    }
+
+    Ok(())
+}