docs: add command-v example pair

[pollychen-lab]

Summary

• add a runnable standards example pair for command -v vs which
• document the pair in the standards examples table

Closes #8.

Validation

• git diff --check
• shellcheck --severity=warning examples/standards/11-command-v-not-which.good.sh in alpine:3.20 container
• shellcheck --severity=style --format=gcc examples/standards/11-command-v-not-which.bad.sh in alpine:3.20 container: reports SC2230
• shfmt -d examples/standards/11-command-v-not-which.good.sh examples/standards/11-command-v-not-which.bad.sh in alpine:3.20 container
• ./tools/check-bad-examples.sh in alpine:3.20 container
• bats tests/examples.bats in alpine:3.20 container

[gemini-code-assist]

Code Review

This pull request introduces a new standard example demonstrating why command -v should be preferred over which for checking command availability, updating the README table accordingly. The feedback suggests renaming the example files and README entry to use sequential numbering (changing 12 to 11 as 11 was skipped) and removing the redundant stderr redirection (2>&1) from the command -v check in the good example.

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[posidoni]

Thank you for your contribution. I will review it carefully and come back with my feedback soon.

[posidoni]

Thanks for the clean contribution! This closes #8 exactly per the contract in CONTRIBUTING.md — the command -v vs which example pair is correct, safe, and both gemini-code-assist nits (file renumbering, redundant 2>&1) are already addressed. CI is green on all required checks. Merging now.

[posidoni]

Merged — thank you for the contribution, @pollychen-lab! This is exactly the kind of clean, well-tested example the project needs. Appreciate you taking the time.