fix: add fail-fast probe for base branch in ship step 12

[Jasperc2024]

/ship Step 12 currently computes BASE_VERSION with:

BASE_VERSION=$(git show origin/<base>:VERSION ... || echo "0.0.0.0")

If origin/ is missing/unfetched/unresolvable (offline, detached HEAD, misconfigured remotes, etc.), the command fails and /ship silently falls back to 0.0.0.0, which can produce an incorrect version bump baseline.

What changed

Added a fail-fast guard before reading origin/:VERSION:
git rev-parse --verify origin/ must succeed, otherwise /ship prints a clear error and exits.
Left the existing parsing/trimming logic intact for the happy path.
Regenerated skill docs after updating the .tmpl file.

Why this is better

Avoids silently selecting an invalid baseline (0.0.0.0) that can cause version drift.
Produces an actionable error message that tells the user how to fix it (git fetch origin / verify base branch).

Testing

bun test
bun run gen:skill-docs --host all

This addresses the TODO item: “/ship Step 12 BASE_VERSION silent fallback to 0.0.0.0 when git show fails”.

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[Copilot]

Pull request overview

This PR updates the /ship Step 12 version-bump instructions to fail fast when the expected base ref (origin/<base>) can’t be resolved, preventing a silent fallback to 0.0.0.0 that could lead to an incorrect version baseline.

Changes:

• Added a guard (git rev-parse --verify origin/<base>) that errors and exits if the base ref is missing/unresolvable.
• Regenerated the rendered skill documentation after updating the template.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
ship/SKILL.md.tmpl	Adds a fail-fast base-ref resolution check before computing BASE_VERSION in Step 12.
ship/SKILL.md	Regenerated rendered docs reflecting the Step 12 fail-fast check.

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[garrytan]

Thanks @Jasperc2024 — your fix shipped in v1.30.0.0 (#1391) with credit in the CHANGELOG. Closing since it's already on main. Appreciate the contribution.