feat: Phase 1 code-agent capabilities — file_edit, git tools, parallel execution, tool output preview

New tools:
- file_edit: surgical line-range or pattern edits (yolo only)
- git_status: show changed files (read-only)
- git_diff: show unstaged/staged diff (read-only)
- git_log: show recent commits (read-only)

Agent improvements:
- parallel execution for independent read-only tool calls
- mutating tools (file_write, file_edit, shell_run) run sequentially
- tool output preview in logs with [output] marker

TUI improvements:
- tool cards now show truncated output preview (up to 8 lines)
- mergeToolLog updates placeholder cards with results
- toolStatusText keeps activity bar single-line

Documentation:
- README updated with git tools and file_edit descriptions

internal/tools/file.go

@@ -81,6 +81,7 @@ type FileWriteTool struct {
 func NewFileWriteTool(workingDir string) *FileWriteTool {
 	return &FileWriteTool{workingDir: workingDir}
 }
+func (*FileWriteTool) Mutates() bool { return true }
 
 func (t *FileWriteTool) Definition() llm.Tool {
 	return llm.Tool{