test(shipping): answer the shipping tests from one fake GitHub

[michael-denyer]

• Add watch-pr/shipping.test-helper.ts with fakeGitHub(pullRequest, argv), one fake of gh api graphql that both shipping test files call. A two-row table holds the contract. Each row names a mutation's required input field and the pull request field the mutation clears.
• The fake answers the way the endpoint does. A query gets back only the fields it selects. The fake refuses a mutation unless the mutation declares $id as ID!, names a known mutation and its input field, and carries the pull request's node id. The fake ignores whitespace.
• shipping.test.ts and shipping-cli.test.ts drop their own dispatchers and call the fake. That replaces the two guards fix(shipping): use GitHub dequeue input id #239 added, which compared the query text byte for byte, and removes a .calls log that no test read.

Why

The guards from #239 catch the dequeue bug. They pass the same wrong-field mistake on disablePullRequestAutoMerge, and they fail a query that differs only in spacing.

Verification

All of these ran on this branch, rebased onto main at fee271b after #239 merged.

• Six source mutations of shipping.ts, each run against bun test shipping. The fix(shipping): use GitHub dequeue input id #239 guards classify 2 of 6 correctly. The fake classifies 6 of 6. Four invalid queries fail, and the unmodified query and the reformatted query pass.
• Two mutations added after review also classify correctly. An inspect query that drops the pull request's own id fails, and a dequeue mutation with an operation name passes.
• bun test orch watch-pr reports 225 pass, 0 fail.
• bun run typecheck prints no diagnostics.
• bunx prettier@3.6.2 --check passes on the three changed files.
• bun tools/generate.mjs --check reports 81 generated files current.

Open risk

The contract table is a hand-kept copy of GitHub's schema, so no test fails if GitHub renames a field. The helper's header comment has the introspection command that regenerates the table.