bastille-jail-provisioning: add FreeBSD-adapted script safety patterns

Merge in a checklist-style "provisioning script safety" section, adapted
from wshobson/agents' bash-defensive-patterns skill (fetched directly
from GitHub, MIT licensed) rather than trusted from its catalog summary.

Covers: shell-choice caution (#!/bin/bash isn't guaranteed on FreeBSD;
bash is a package at /usr/local/bin/bash, not base), a contextual
error-handling philosophy instead of a blanket `set -Eeuo pipefail`,
trap-based cleanup/logging for scripts interrupted mid-jail-creation
(without auto-rollback, which can worsen the known destroy/create IP
churn issue), jail-name/IP input validation before destructive `bastille`
commands, and FreeBSD `mktemp`/`sed -i`/`date` syntax differences from
GNU. Deliberately dropped the upstream's interactive `rm -rI` cleanup
pattern (wrong for unattended SSH automation) and did not import
retry/locking/ShellCheck-gate advice that wasn't actually present in the
real upstream files. Cross-references remote-shell-quoting-safety instead
of duplicating its nested-shell-quoting content.

Also documents the /usr/local/bastille vs /www/bastille bastille_prefix
split already present across this fleet's hosts (gringo/staging vs
granja), which the new script-safety guidance assumes readers already
know not to hardcode.

Full worked patterns and code examples live in the new
references/script-safety.md; SKILL.md keeps the load-bearing summary.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

skills/bastille-jail-provisioning/SKILL.md

@@ -265,3 +265,63 @@ exist under that name in the FreeBSD ports tree) — and remember Valkey's
 ACL means any code connecting to it needs an explicit `AUTH` step, not just
 `connect()`; a `connect()`-only implementation will silently fail every
 subsequent command with `NOAUTH` against this fleet's standard Valkey setup.
+
+## Bastille filesystem prefix varies by host — check before hardcoding a path
+
+Don't assume `/usr/local/bastille/jails/<name>/root/...` — that's only
+the convention where `bastille.conf` uses the default `bastille_prefix`
+(confirmed on gringo and staging). A host with a custom
+`bastille_prefix` (e.g. `granja`, where storage was repointed at a
+dedicated `wwwpool` with `bastille_prefix=/www/bastille`) puts jails
+under `/www/bastille/jails/<name>/root/...` instead — writing to the
+`/usr/local/bastille/...` path on that host silently succeeds (creates a
+new, wrong directory) rather than erroring, since nothing stops you from
+writing to an arbitrary path. **Check the host's actual
+`bastille_prefix`** (`grep bastille_prefix /etc/rc.conf` or
+`bastille.conf`) before hardcoding either path in a script.
+
+## Provisioning script safety
+
+Applies to any shell script written to automate part of this
+provisioning flow (not the one-off interactive commands above). Full
+worked patterns, including FreeBSD-specific `mktemp`/`sed`/`date` syntax,
+a trap-based cleanup template, and jail-name/IP input validation, live in
+`references/script-safety.md` — read that before writing a new
+provisioning script. Summary of the load-bearing points:
+
+- **Don't assume `#!/bin/bash` exists.** FreeBSD base ships `/bin/sh`
+  (POSIX-ish ash), not bash — bash is a package that installs to
+  `/usr/local/bin/bash`, never `/bin/bash`, and many freshly cloned jails
+  won't have it installed at all. Default to `#!/bin/sh` and POSIX
+  constructs unless bash-only features are genuinely needed and you've
+  confirmed bash is present on the target.
+- **Error handling is contextual, not a blanket `set -Eeuo pipefail`.**
+  Turn on `set -eu` for the whole script, but explicitly tolerate steps
+  that are allowed to fail (e.g. an already-installed package, an
+  optional service restart) with a commented `|| true` rather than either
+  aborting the whole provisioning run on a non-fatal hiccup or suppressing
+  errors wholesale. `-o pipefail`/`-E` are bash-only and unavailable under
+  `/bin/sh`.
+- **Trap cleanup, don't trap auto-rollback.** A script that dies partway
+  through jail creation should log exactly which step it reached (jail
+  created? DB granted? vhost written?) so a human can clean up — see the
+  create/destroy IP-alias churn gotcha above for why an automatic
+  `bastille destroy` inside a failure-path trap can make things worse, not
+  better.
+- **Validate jail names and IPs before they reach a destructive `bastille`
+  command.** `bastille destroy <name>` has no confirmation prompt.
+  Validate with plain POSIX `case`/`[` pattern matching (works under both
+  `/bin/sh` and bash) before any value — especially one derived from
+  outside the script — is interpolated into a command.
+
+For quoting content through nested shell layers (`ssh` → `bastille cmd`
+→ `su`), use the separate `remote-shell-quoting-safety` skill — that
+problem is not duplicated here.
+
+Provenance: the script-safety patterns above are adapted from
+wshobson/agents' `bash-defensive-patterns` skill
+(https://github.com/wshobson/agents,
+`plugins/shell-scripting/skills/bash-defensive-patterns`, MIT licensed,
+fetched 2026-08-15) — rewritten for FreeBSD/Bastille where upstream
+assumed GNU/Linux; see `references/script-safety.md` for exactly what was
+kept, rewritten, or dropped.