refactor(cmd): re-enter child modes via environment instead of hidden flags

Assisted-by: GLM 5.3 Flash

cmd/gasm/debug_linux.go

@@ -24,15 +24,30 @@ function in a traced subprocess (ptrace), then provides a REPL for
 single-stepping, breakpoints, register and memory inspection.
 
 REPL commands:
-  break <label|addr>   set a breakpoint at a label or absolute address
-  step [n]             single-step n instructions (default 1)
-  continue             run until next breakpoint or exit
-  regs                 print general-purpose registers
+  break <label|addr> [if <reg> <op> <val>]
+                       set a breakpoint, optionally conditional on a
+                       register comparison (reg-reg or reg-immediate)
+  delete <label|addr>  remove a breakpoint
+  info break           list all breakpoints
+  step [n], s          single-step n instructions (default 1)
+  next, n              step over a CALL
+  finish, fin          run until the function returns
+  continue, c          run until a breakpoint, watchpoint or exit
+  disas [n], u         disassemble n instructions at PC
+  regs                 print general-purpose and vector registers
+  where                show source line and nearest label at PC
+  stack                show stack near RSP (return address + ABI0 args)
+  bt, backtrace        backtrace (current frame + return address)
   x [addr] [len]       hex-dump memory (default: current PC, 64 bytes)
-  labels               list function labels and offsets
-  quit                 kill the debuggee and exit
+  w <addr> <val...>    write bytes to memory
+  set <reg> <value>    set a register
+  watch <addr> [r|w] [size]
+                       set a hardware watchpoint (write by default)
+  unwatch [<slot>]     clear one watchpoint, or all without an argument
+  labels, l            list function labels and offsets
+  help, h, ?           show command help
+  quit, q              kill the debuggee and exit
 `)
-	target := fs.Bool("target", false, "") // hidden: debuggee subprocess mode
 	funcName := fs.String("func", "", "function to debug")
 	argsFile := fs.String("args", "", "file containing the ABI0 argument block")
 	bufSpec := fs.String("buf", "", "buffer specification: name:size:pattern[,name:size:pattern...] where pattern is zero, ones, seq, or hex")
@@ -42,10 +57,10 @@ REPL commands:
 	fs.Parse(args)
 
 	// --- Debuggee mode (internal, spawned by the debugger) ---
-	if *target {
+	if os.Getenv("GASM_DEBUG_TARGET") != "" {
 		tmpDir := os.Getenv("GASM_DEBUG_TMP")
 		if tmpDir == "" || fs.NArg() < 1 || *funcName == "" || *argsFile == "" {
-			fmt.Fprintln(os.Stderr, "gasm debug --target: internal mode")
+			fmt.Fprintln(os.Stderr, "gasm debug: internal debuggee mode")
 			return 2
 		}
 		if err := debug.RunTarget(fs.Arg(0), *funcName, *argsFile, tmpDir); err != nil {