feat(commands): add /edit help subcommand

/edit help, /edit --help, /edit -h all display full usage with examples.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/commands/cmd_edit.go

@@ -1,13 +1,39 @@
 package commands
 
-import "context"
+import (
+	"context"
+	"strings"
+)
+
+const editHelp = `Usage:
+  /edit <file>                — view file with line numbers
+  /edit <file> <N> <text>    — replace line N with text
+  /edit <file> +<N> <text>   — insert after line N
+  /edit <file> -<N>          — delete line N
+  /edit <file> -m """        — write full content (multi-line)
+  <content>
+  """
+
+Examples:
+  /edit main.go              — view main.go
+  /edit main.go 5 func foo() — replace line 5
+  /edit main.go +10 // note  — insert after line 10
+  /edit main.go -3           — delete line 3
+  /edit README.md -m """
+  # Title
+  Hello world
+  """                        — overwrite file`
 
 func editCommand() Definition {
 	return Definition{
 		Name:        "edit",
 		Description: "View or edit files (works in both chat and command mode)",
-		Usage:       "/edit <file> [operation]",
+		Usage:       "/edit <file> [operation] | /edit help",
 		Handler: func(_ context.Context, req Request, rt *Runtime) error {
+			arg := strings.TrimSpace(strings.TrimPrefix(strings.TrimSpace(req.Text), "/edit"))
+			if arg == "help" || arg == "--help" || arg == "-h" {
+				return req.Reply(editHelp)
+			}
 			if rt == nil || rt.EditFile == nil {
 				return req.Reply(unavailableMsg)
 			}