Add example pair for readonly constants (rule 14)

[posidoni]

Context

reference/shell-standards.md rule 14
("Make constants readonly") has no runnable pair under
examples/standards/ proving it, unlike most of the
other 17 non-negotiables. Follow the contract in
CONTRIBUTING.md.

Plan

1. Add examples/standards/12-readonly-constants.good.sh:
   • Declare a value that must never change with readonly (e.g.
     readonly max_retries=3), then use it.
   • Self-contained, no arguments, exits 0, clean under
     shellcheck --severity=warning and shfmt.
2. Add examples/standards/12-readonly-constants.bad.sh:
   • Declare the same "constant" as a plain variable, then reassign it later
     in the script (simulating an accidental shadow/typo elsewhere) to show
     nothing protects it.
   • Safe to run. # expect-shellcheck: none — ShellCheck has no code for
     "this should be readonly"; it's a style-guide-only pitfall.
3. Add a row to the table in examples/standards/README.md.

Verify

task ci

must stay green — tools/check-bad-examples.sh asserts the .bad.sh
directive is present, and tests/examples.bats runs the new .good.sh.

A small ask

If you used an AI coding agent for this PR, feel free to say so — add a line
to the PR description like:

Authored by <your name/handle>, implemented using <model name>.

Either way is genuinely welcome here.