fix: resolve ShellCheck violations in bin/ and setup scripts

[superdav42]

Summary

The daily quality sweep (2026-03-25) identified 9 ShellCheck findings across 6 shell scripts. These are tracked as quality-debt in issue #400.

Findings

Errors (3)

• bin/deploy.sh:SC2015 — A && B || C is not if-then-else; C may run when A is true
• bin/docker-up.sh:SC2181 — Check exit code directly with if ! mycmd;, not indirectly with $?
• bin/mailpit.sh:SC2275 — Command name starts with =. Bad line break?

Warnings (6)

• bin/docker-down.sh:SC2148 — Add a shebang or a shell directive
• bin/install-wp-tests.sh:SC2086 — Double quote to prevent globbing and word splitting
• bin/install-wp-tests.sh:SC2046 — Quote this to prevent word splitting
• bin/install-wp-tests.sh:SC2006 — Use $() notation instead of legacy backticks
• inc/site-exporter/mu-migration/bin/test.sh:SC2086 — Double quote to prevent globbing and word splitting
• setuptest.sh:SC2148 — Add a shebang or a shell directive
• setuptest.sh:SC2086 — Double quote to prevent globbing and word splitting

Acceptance Criteria

• All 9 ShellCheck violations resolved (run shellcheck bin/*.sh setuptest.sh inc/site-exporter/mu-migration/bin/test.sh to verify)
• No new ShellCheck violations introduced
• All scripts retain their existing functionality

[superdav42]

All ShellCheck violations have been resolved:

• PR fix: resolve ShellCheck violations in bin/ scripts and setuptest.sh #440 fixed SC2015 (bin/deploy.sh), SC2181 (bin/docker-up.sh), SC2275 (bin/mailpit.sh), SC2148 (bin/docker-down.sh, setuptest.sh), SC2086/SC2046/SC2006 (bin/install-wp-tests.sh, setuptest.sh)
• PR fix: SC2086 quote $BEHAT_TAGS in mu-migration/bin/test.sh #444 fixed SC2086 (inc/site-exporter/mu-migration/bin/test.sh)

Verified: shellcheck bin/deploy.sh bin/docker-down.sh bin/docker-up.sh bin/install-wp-tests.sh bin/mailpit.sh setuptest.sh inc/site-exporter/mu-migration/bin/test.sh exits 0 with no violations.