ci(regen): seed changelog under [Unreleased], drop version bump

[zfarrell]

The regenerate workflow auto-bumped pyproject.toml (patch) and minted a dated ## [X.Y.Z] CHANGELOG section per run. That mints a version that may never publish (e.g. 0.4.1 sat bumped-but-unreleased until #120 rolled #118 into it).

A regen is just a set of changes; which release ships them — and the bump kind — is scripts/release.sh prepare's job. This makes regen seed its note under ## [Unreleased] and stop touching the version.

• Removes the version-bump step; regen no longer changes pyproject.toml.
• Seeds the regen title under [Unreleased] (prepends to ### Changed, creating it if absent; idempotent).
• Drops packageVersion from the generator call — unused (version reads from package metadata).
• check-release.py is a no-op when the version is unchanged, and regen still touches CHANGELOG.md, so the wheel build/import smoke test still runs. Updated the stale comments that assumed a version bump.

[claude]

super nit: the idempotency guard uses a substring check (bullet in body), so a title that is a prefix/substring of an already-listed bullet (e.g. seeding - Add foo when - Add foobar already exists) would be treated as already-present and silently skipped, dropping the regen note. A line-exact match would be more robust, e.g. if any(line.strip() == bullet for line in body.splitlines()). Minor since regen titles are typically long/unique. (not blocking)

[zfarrell]

Good catch — switched to a line-exact match (any(line.strip() == bullet ...)) so a prefix/substring title can't be silently dropped. Applied here and in the matching sdk-rust PR.

[claude]

Reviewed. Logic is correct and consistent with the release tooling (update_changelog.py rolls the [Unreleased] body into the version section on release). One non-blocking super nit left inline.

[claude]

Reviewed: regen now seeds notes under [Unreleased] and drops the version bump. Logic traced against the current CHANGELOG and the release flow (update_changelog.py/release.sh consume [Unreleased]); check-release.py is a no-op for unchanged versions, so regen PRs still pass while the wheel smoke test runs on the CHANGELOG touch. Idempotency uses the line-exact match. LGTM.