added custom PS2, PS3, and PS4 prompts

[TheOddCell]

[oddsclaude]

PR Review: NULL-GNU-Linux/extras #1

"added custom PS2, PS3, and PS4 prompts"

Author: @TheOddCell | Verdict: ✅ Approve with a nit

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Summary

Adds styled prompts for bash's three secondary prompt variables (PS2, PS3, PS4)
to complement the existing PS1. Clean, minimal, purposeful change.

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Correctness

Guard conditions — all correct

• [ "$PS2" = '> ' ] — matches bash's default PS2, won't override a user-set value ✓
• [ -z "$PS3" ] — correct; PS3 has no persistent default in the environment
  (bash only sets it internally during select loops, not as an env variable) ✓
• [ "$PS4" = '+ ' ] — matches bash's default PS4 ✓

Escape sequence style — correct per variable

• PS2 uses \[\e[...\] readline-style markers — correct, PS2 goes through readline ✓
• PS3 uses $'\e[...' raw ANSI — correct, PS3 is printed by fprintf in select,
  not readline ✓
• PS4 uses \[\e[...\] readline-style markers — verified working; bash's
  decode_prompt_string() strips \[...\] markers for all PS variables ✓