ci: keep uv.lock in sync on the release-please branch

[kartojal]

Every release-please merge bumps the version in pyproject.toml but leaves uv.lock recording the previous one, so uv sync --locked fails Static Checks on main until someone regenerates the lockfile by hand (done manually after 0.1.0, 0.2.0 (#207), and 0.3.0).

Fix: a sync-lockfile job runs after release-please, checks out the release PR branch, runs uv lock, and commits the result, so the release PR merges self-contained. A workflow step instead of release-please extra-files because uv lock rewrites the lockfile wholesale (an x-release-please-version annotation would not survive) and the toml updater would need a fragile jsonpath into the [[package]] array. The commit step no-ops when the lock is already current, and publish does not depend on this job, so a sync failure can never block a publish.

Release-please also writes semver-style prerelease versions (0.3.0-b2) into pyproject.toml, and the __version__ fallback in src/polymarket/version.py is not updated at all, which is the other half of the recurring release: canonicalize python beta version commits. Worth its own ticket rather than bundling it here.

Closes DEV-483

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Note

Low Risk
CI-only workflow changes with no runtime or auth impact; publish remains decoupled from lockfile sync.

Overview
Fixes uv sync --locked failures on main after release-please bumps pyproject.toml without updating uv.lock.

The Release Please workflow now uses a concurrency group and exposes prs_created. When a release PR is opened, a new sync-lockfile job checks out the release-please branch for polymarket-client, runs uv lock, and pushes a commit only if the lockfile changed. Publish to PyPI still depends only on release-please and releases_created, so a lock sync failure cannot block publishing.

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