Add example pair for trap-quoting (SC2064)

[posidoni]

What's missing

reference/shell-standards.md rule 9
("Clean up with trap on EXIT") explains that a trap command must be
single-quoted so the cleanup expression is evaluated when the signal fires,
not when trap is registered. Unlike every other rule in that doc, rule 9
has no runnable pair under examples/standards/
proving it.

Verified locally with ShellCheck 0.11.0:

$ shellcheck --severity=style --format=gcc t.sh
t.sh:4:14: warning: Use single quotes, otherwise this expands now rather than when signalled. [SC2064]

What to do

Add examples/standards/11-trap-single-quote.good.sh and .bad.sh,
following the contract in
CONTRIBUTING.md:

• .good.sh — creates a temp dir with mktemp -d, sets
  trap 'rm -rf "$tmp"' EXIT (single-quoted), self-contained, exits 0,
  clean under shellcheck --severity=warning and shfmt.
• .bad.sh — the same pattern but double-quoted
  (trap "rm -rf $tmp" EXIT), safe to run, with
  # expect-shellcheck: SC2064.
• Add a row to the table in examples/standards/README.md.

Verify

task ci

must stay green — tools/check-bad-examples.sh asserts the new .bad.sh
actually triggers SC2064, and tests/examples.bats runs the new
.good.sh.