fix(scripts): refuse a PR that modifies or deletes a changeset it did not add (#17712)

A changeset filename is content-free, so a collision looks like nothing:
the changesets default word-pair names were designed for one human running
the CLI at a time, and this repository runs many agents in parallel drawing
from the same small name space. Overwriting somebody else's changeset
produces a perfectly valid changeset file, so every parse-shaped gate stays
green on both sides -- the sibling PR's release note is silently replaced,
its own CI never re-runs, and the loss surfaces at release time in the
generated CHANGELOG with the authoring PR long merged.

check-empty-changeset.mjs now answers a second question in the same run:
a `.changeset/*.md` that exists on the merge base and was not added by this
PR may be neither modified nor deleted. "Added by this PR" is the file's
absence on the merge base, which is what git's status letters already say,
so the rule is content-blind and legacy names are untouched. Rename
detection is off for this pass (the opposite of rule 1's `AMR`): renaming
somebody else's changeset deletes their release note at its path, and with
detection on that deletion is folded into an `R` row and disappears.

Wired into check-empty-changeset.mjs rather than a new script so it inherits
the `changeset-check` job's merge base, its `changeset-release/main`
exemption -- `changeset version` deletes every changeset, so the release PR
would be structurally unsatisfiable otherwise -- and its self-test's place
in lint.yml, with no workflow diff at all.

Claude-Session: https://claude.ai/code/session_012GKcPZbMoGq7WPzKLfRBTU
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude