ci: record each published release in Linear Releases

[PabloPardoGarcia]

Adds a linear-release job to the release workflow: after the PyPI publish succeeds, it syncs the new version into the Python SDK release pipeline in Linear via linear/linear-release-action (pinned to v0.17.2).

Attribution needs no convention change: the CLI detects the squash-merge PR numbers (Title (#65)) and Linear resolves them to the issues those PRs are linked to via branch names. Issues then show which SDK version shipped them, and "merged but unreleased" becomes a filterable state instead of tribal knowledge.

Safe to merge before the pipeline exists: the job skips with a warning while the LINEAR_ACCESS_KEY secret is unset (same pattern as DOCS_DISPATCH_TOKEN).

To activate:

1. Create a continuous pipeline "Python SDK" in Linear → Settings → Releases (releases require the Business plan).
2. Generate its access key and add it as the LINEAR_ACCESS_KEY repo secret.

Note: on the first sync only the current commit is scanned (no prior release bounds the range), so the first recorded release will attach little — the baseline for every later release.