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/tools.go

@@ -19,6 +19,18 @@ type Tool interface {
 	Execute(ctx context.Context, args json.RawMessage) (string, error)
 }
 
+// MutatingTool is optionally implemented by tools that modify external state.
+// It is used by the agent to determine whether tool calls can run in parallel.
+type MutatingTool interface {
+	Mutates() bool
+}
+
+// IsMutating reports whether tool implements MutatingTool and returns true.
+func IsMutating(tool Tool) bool {
+	mutating, ok := tool.(MutatingTool)
+	return ok && mutating.Mutates()
+}
+
 type Registry struct {
 	tools   map[string]Tool
 	primary []string
@@ -66,30 +78,39 @@ func ReadOnlyBuiltins(workingDir string) *Registry {
 	fileList := NewFileListTool(workingDir)
 	fileSearch := NewFileSearchTool(workingDir)
 	codeSymbols := NewCodeSymbolsTool(workingDir)
+	gitStatus := NewGitStatusTool(workingDir)
+	gitDiff := NewGitDiffTool(workingDir)
+	gitLog := NewGitLogTool(workingDir)
 
-	registry := NewRegistry(fileRead, fileList, fileSearch, codeSymbols)
+	registry := NewRegistry(fileRead, fileList, fileSearch, codeSymbols, gitStatus, gitDiff, gitLog)
 	registry.RegisterAlias("file.read", fileRead)
 	registry.RegisterAlias("file.list", fileList)
 	registry.RegisterAlias("file.search", fileSearch)
 	registry.RegisterAlias("code.symbols", codeSymbols)
+	registry.RegisterAlias("git.status", gitStatus)
+	registry.RegisterAlias("git.diff", gitDiff)
+	registry.RegisterAlias("git.log", gitLog)
 	return registry
 }
 
 func Builtins(workingDir string) *Registry {
 	registry := ReadOnlyBuiltins(workingDir)
 	fileWrite := NewFileWriteTool(workingDir)
+	fileEdit := NewFileEditTool(workingDir)
 	shellRun := NewShellRunTool(workingDir)
 	registry.Register(fileWrite)
+	registry.Register(fileEdit)
 	registry.Register(shellRun)
 	registry.RegisterAlias("file.write", fileWrite)
+	registry.RegisterAlias("file.edit", fileEdit)
 	registry.RegisterAlias("shell.run", shellRun)
 	return registry
 }
 
 func AgentInstructions(workingDir string, yolo bool) string {
 	searchInstructions := webSearchInstructions()
-	readOnlyTools := "file_list, file_read, file_search, code_symbols, web_search, web_fetch"
-	yoloTools := "file_list, file_read, file_search, code_symbols, file_write, shell_run, web_search, web_fetch"
+	readOnlyTools := "file_list, file_read, file_search, code_symbols, git_status, git_diff, git_log, web_search, web_fetch"
+	yoloTools := "file_list, file_read, file_search, code_symbols, file_write, file_edit, shell_run, git_status, git_diff, git_log, web_search, web_fetch"
 	if yolo {
 		return fmt.Sprintf(`Local project context:
 - The project working directory is %q.
@@ -99,7 +120,9 @@ func AgentInstructions(workingDir string, yolo bool) string {
 - When the user asks to inspect, list, search, or read project files, use file_list, file_search, and file_read instead of saying you cannot access local files.
 - When the user asks about code structure, symbols, functions, methods, types, or definitions in Go files, use code_symbols before reading whole files.
 - When the user asks to run, execute, test, check, debug, or inspect a shell command, use shell_run. This includes local commands written in backticks, such as go, make, git, pwd, ls, ps, and similar terminal commands.
+- When editing existing files, prefer file_edit for small or localized changes; use file_write only for creating files, replacing whole files intentionally, or appending.
 - Do not use file tools as a substitute for a command execution request.
+- When the user asks about changed files, diffs, or git history, use git_status, git_diff, or git_log instead of shell_run.
 - Do not start interactive or long-running sessions. Prefer bounded, non-interactive local commands with clear completion conditions.
 - %s
 - Tool paths should normally be relative to the project working directory.
@@ -113,6 +136,7 @@ func AgentInstructions(workingDir string, yolo bool) string {
 - When the user asks to inspect, list, search, or read project files, use file_list, file_search, and file_read instead of saying you cannot access local files.
 - When the user asks about code structure, symbols, functions, methods, types, or definitions in Go files, use code_symbols before reading whole files.
 - Shell command execution is disabled in this mode. When the user asks to run, execute, test, check, debug, or inspect a shell command, do not use file tools as a substitute; explain that they need to restart agentu with --yolo to enable shell_run.
+- When the user asks about changed files, diffs, or git history, use git_status, git_diff, or git_log instead of shell_run.
 - %s
 - Tool paths should normally be relative to the project working directory.
 - Available tools: %s.`, workingDir, searchInstructions, readOnlyTools)