fix(guard): Gate 1 of the pre-release skill has been checking an empty directory

[noahgift]

check_package_includes.sh is the CB-510 guard: a Cargo.toml exclude pattern
can strip an include!() target from the published crate while git still tracks
it, so the crate compiles in-tree and fails for everyone installing from
crates.io. That has shipped twice -- models/ matching src/models/, and an
unanchored "tests/" dropping 443 files from published aprender-serve in 0.63.0.

It scanned src/ and packaged -p aprender: the PRE-MONOREPO layout. After
consolidation the root src/ holds 2 files with zero include!(), while 1798
live under crates/. So it reported, truthfully and uselessly:

OK: All 0 include!() files are included in cargo package

Zero of zero, exit 0, for every release since the consolidation -- while being
Gate 1 of .claude/skills/pre-release/SKILL.md.

It now enumerates publishable workspace crates from cargo metadata, resolves
every include!() against the including file's directory, and diffs against that
crate's OWN cargo package --list. 10 crates, 1539 include targets. A vacuity
assertion fails below 100 targets, so the empty-scan mode cannot return.

RESULT: the tree is CLEAN. Every one of the 1539 targets survives packaging.

Three bugs of my own, each caught by a mutation that refused to turn red. Worth
recording because each produced a confident wrong answer:

1. cargo package inside while read ... <<< "$pkgs" consumed the heredoc on
   stdin. The scan silently dropped a crate (11 -> 10) AND compared one crate's
   include targets against another crate's listing, inventing a CB-510 violation
   on src/bench/backend.rs. Fixed with < /dev/null.
2. One forked grep -qxF per target -- 922 for aprender-serve alone -- treating
   ANY non-zero as "not packaged". grep exits >1 on ERROR, and a forked grep can
   die under load, so the guard named a different innocent file on each run.
3. Replacing that with python, I put a heredoc script AND a <<< data
   redirection on the same call. Last redirection wins, so python received the
   include list as its SCRIPT and printed nothing. The guard then PASSED a
   mutation that provably dropped a file from the package -- I had written
   another gate that cannot fail, inside the fix for a gate that cannot fail.
   Both inputs now go through explicit files.

Mutation-verified, with the mutation itself proven to engage first
(cargo package --list | grep -c goes 1 -> 0 before the guard is consulted):

• exclude "src/bench/" -> RED, 27 files named
• scan a nonexistent dir -> RED on vacuity
• restored -> GREEN

Wired into the merge-blocking guard block. bashrs 0 errors; the three embedded
python fragments moved to scripts/lib/ because bashrs parses an inline heredoc as
shell (8 phantom SC1007s from python assignments, 2 SC1078s from nested quotes
across a line continuation).

Refs #2481, #2474

Co-Authored-By: Claude Opus 5 noreply@anthropic.com