The upgrade command tests had weak assertions that could pass vacuously,
and changelog validation coverage was lost when tests were deleted from
upgrade-flow.test.ts.

Upgrade test improvements:
- Verify renderTasks receives specific task titles instead of just
  toHaveBeenCalled()
- Assert removal task is absent when no deps to remove (direct mock
  inspection pattern)
- Add tests for removed packages display in confirmation prompt and
  generated markdown instructions

New changelog-schema.test.ts (~290 lines):
- Validates changelog.json schema: allowed fields, required fields,
  URL/version/date formats, semver deps, base64 steps, dependenciesMeta
- Uses semver.validRange() instead of a permissive regex for version
  validation — the regex accepted trailing garbage like 1.0.0!@#$%
- Precondition assertions on required-field loops prevent vacuous passes
  (e.g. if all releases lost their features/fixes arrays, the test would
  still pass green without the precondition)
- Optional fields (dependenciesMeta, removeDependencies) intentionally
  have no preconditions — a valid changelog may have zero instances

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

- Replace fragile `mock.calls[0]?.[0]` traversal with
  `expect.not.arrayContaining` per kdaviduik's suggestion; the
  built-in matcher is resilient to call-order changes and removes
  the need for the inline `{title: string}` cast
- Add negative test for `generateUpgradeInstructionsFile` asserting
  the "Removed packages" section is omitted when there are no removals
- Add edge-case test for both `displayConfirmation` and
  `generateUpgradeInstructionsFile` when a release has only removals
  and no features/fixes — covering the new UX path noted by fredericoo
- Extract `assertDefined<T>` type assertion helper in
  `changelog-schema.test.ts` to replace the 6x repeated
  `expect(release).toBeDefined(); if (!release) continue;` pattern;
  after calling `assertDefined`, TypeScript narrows the type so the
  manual guard is no longer needed
- Hoist `getChangelog()` into `beforeAll` in `changelog-schema.test.ts`
  to parse the static JSON once instead of once per test (~9 calls)
- Update changeset wording per kdaviduik's nit

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

When `allRemovedPackages` is present but `fixes` is absent, the prior
section (h1/breaking/features) already ends with `\n----\n`. The old
code unconditionally prepended `\n----\n\n` before "## Removed packages",
producing a `----\n\n----` double horizontal rule in the output.

Fix: conditionally add the separator only when `fixesMd.length > 0`
(fixes emits no trailing separator, so we supply one) vs. `'\n'` when
fixes is absent (prior section already terminated with `----\n`).

Also replace silent `if (!item) continue` / `if (!step) continue`
guards in changelog-schema.test.ts with `assertDefined()` so null/
undefined entries in changelog.json cause an explicit test failure
rather than a silent skip that masks malformed data.