Migrate operational review documents from Show to built-in <If>

[taras]

Problem

The review system still carries .reviews/components/Show.md, and seven
operational review documents use it for conditional output. Executable.md now
has the native structural <If> directive, so this local standard-library
component duplicates engine behavior and lets the operational documents drift
from the documented language.

The original release dependency is obsolete. .github/workflows/review.yml
prepares and builds the checked-out revision and runs ./dist/xmd; it no longer
installs the latest published binary. <If> is also present in v0.7.0.

Scope

Replace Show directly with If in these seven documents:

• .reviews/components/CommentReview.md
• .reviews/components/Finding.md
• .reviews/components/OxlintSummary.md
• .reviews/components/ReleaseSpecWarning.md
• .reviews/components/UnusedInDiff.md
• .reviews/policies/ExtraneousCodePolicy.md
• .reviews/policies/RepoCleanupPolicy.md

For each block, rename when to condition and preserve its body and authored
whitespace. The current expressions already produce booleans. None has a
fallback, so no <Else> is needed.

Also:

• delete .reviews/components/Show.md;
• remove the Show.md fixture copy from
  packages/core/tests/unused-in-diff.test.ts;
• update the stale Show wording and example in
  specs/code-review-agent-spec.md; and
• add review-infrastructure regressions that execute the migrated operational
  documents and prove native <If> resolution, selected/suppressed output,
  and non-execution of a false branch.

Acceptance

• No Show tag or .reviews/components/Show.md remains in .reviews/.
• No focused test fixture or code-review specification claim still treats
  Show as part of the review component library.
• The migrated documents resolve built-in <If> without a Show or If
  component stub.
• UnusedInDiff preserves both its rendered disclosure/table and its empty
  suppressed output.
• Review-infrastructure tests execute all seven migrated documents without an
  ERROR marker and prove that an unselected branch does not reach Sample.
• Compact selected and suppressed outputs retain their existing whitespace and
  visible text.
• When local review credentials and a model are available, a credentialed local
  review is recorded as integration evidence. Missing credentials or model
  availability skips that evidence; it is not a local test failure.
• The PR's checked-out-source review workflow posts its substantive review
  comment without an ERROR marker before merge.

Out of scope

• changes to <If> or <Else> syntax, validation, evaluation, or truthiness
  semantics, including issue Coerce the <If> condition with JS truthiness #258
• runtime, expansion-engine, component-resolution, provider, or Workspace code
• .github/workflows/review.yml or any other workflow
• dependencies, manifests, lockfiles, generated files, or release behavior
• architecture or executable-MDX specification changes
• renaming unrelated test fixtures that use Show merely as an arbitrary
  component name