plan-feature: verify-citations fails closed on no input

[bert-script]

Why

verify-citations.sh printed All 0 citations resolve. and exited 0 on two inputs that verified nothing:

• an unreadable file argument (a typo in the path), and
• empty input, or input with only comments and blanks.

Phase 1.4 reads exit 0 as "every citation is real", so the gate passed having checked nothing. A consumer's reviewer caught it.

What

• Both cases now exit 2 with a message on stderr. The header comment documents exit 2.
• verify-citations.test.sh gains three cases for them (35 assertions).
• Identical change in skills/plan-feature/scripts/ and plugins/core-skills/skills/plan-feature/scripts/.

Not in this PR: plugins/context-economy/lib/verify-citations.sh has the same fail-open behaviour (checked: empty stdin and a missing file both exit 0 with "All 0 citations resolve."). It is a separate plugin with its own release, so it is left for you to decide.

From Lokale Keuze's copy of this skill (LK-1305).

No plugin.json version bump. This is one of eight parallel PRs from the same consumer, and each bump would collide on the same line. Bump on merge as you prefer.

Test plan

• Suite passes under bash 5 and /bin/bash 3.2; shellcheck --severity=warning clean.
• CI green

🤖 Generated with Claude Code

[bjvle]

SKILL.md should be updated alongside this fix. It still only explains MISSING and says "you may not proceed past Phase 1.4 with a non-zero exit". Exit 2 does block, correctly, but the agent isn't told what it means or what to do next.

E.g. on skills/plan-feature/SKILL.md:198 and plugins/core-skills/skills/plan-feature/SKILL.md:200 the bold sentence becomes:

A MISSING line is a fabricated citation — fix it or drop the claim. Exit 2 means nothing was verified — the input was empty or unreadable — not that a citation is fabricated: fix the pipeline so it feeds the citations you're about to make, and rerun. You may not proceed past Phase 1.4 with a non-zero exit.