Add --base-ref to manually override scan base

[RomainCscn]

--base-ref scans <ref>..HEAD, applies existing path filters, and stores HEAD as the new release baseline.

When provided, it skips automatic release baseline selection for that run. This covers the original first-sync use cases, where no useful prior release commitSha exists yet, and also workflows where the desired lower bound is not a prior release commitSha, such as hotfix branches whose release delta should start at the fork point from an integration branch.

Usage examples:

First-time monorepo onboarding:

linear-release sync --base-ref=<commit-before-backfill> --include-paths="apps/api/**"

Rewritten repo migration:

linear-release sync --base-ref=<new-repo-baseline> --include-paths="apps/api/**"

Hotfix branch delta:

linear-release sync --base-ref="$(git merge-base origin/develop HEAD)"

--base-ref is advanced range control: the ref must be an ancestor of HEAD, and users intentionally own the selected range.

---

Fixes #88.

[linear-code]

LIN-71182

[linear-code]

Looks good. The --base-ref escape hatch is correctly guarded (throws when a reachable baseline already exists), the inspectSingleCommit: false path properly handles baseRef == HEAD as an empty range, and the release-at-zero-commits behavior for baseline establishment is both tested and intentional. No blocking issues found.

[axelniklasson]

Nice! We should add this and any other missing arguments if there are any to https://github.com/linear/linear-release-action as well to ensure parity between the action and the CLI.

[RomainCscn]

Of course, PR incoming!