fix(ci): unreleased changelog subsection clash + PR linkcheck loop

Two CI failures on this PR, both surfaced when the new Unreleased
changelog section landed:

- check (setup.py check --strict): docutils warned "Duplicate
  implicit target name" for "Breaking", "Tooling and CI", and
  "Dependencies" — the Unreleased section had H3 subsections with
  the same names as the 0.3.0 section below, and docutils generates
  anchors from headings. Flatten the Unreleased section to a bullet
  list with bold prefixes; future releases can decide their own
  shape, only the current Unreleased one needs to avoid the clash.

- docs (sphinx-build linkcheck): :pr:`64` resolves to
  github.com/.../pull/64 which threw "Exceeded 30 redirects" — most
  likely GitHub anti-bot bouncing the unauthenticated linkcheck
  request. Ignore github.com/.../pull|issues/\d+ patterns in
  linkcheck_ignore; these references are stable once the PR exists
  and don't need pre-merge linkcheck validation.

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