ci: scope commitlint to PR-introduced commits, fix title suffix check

[MichaelTaylor3d]

Summary

• Scope commitlint on a main-base cut PR to the commits it introduces (commitDepth: 1), not the whole inherited main..develop range — those commits were already linted at their own PRs while still mutable, so re-linting them at cut time adds no information and cannot be satisfied (history is now pinned by gitlinks and rev-pinned deps).
• Keep develop-base PRs linting their full range unchanged — that range is still mutable and its authors can still fix it.
• Fix the Lint PR title step's blind spot: GitHub's squash merge lands "$PR_TITLE (#$PR_NUMBER)" as the commit subject, about 8 characters longer than the title alone. A title within header-max-length: 100 could still produce an over-limit commit subject that nothing checked. Now lints the exact string that lands.

Why two steps instead of one conditional input

${{ cond && 1 || 0 }}-style single-input conditionals are a real trap in Actions expression syntax — 0 and empty string are both falsy, so a wrong form silently disables the gate for every PR. Used two steps with mutually exclusive if: github.base_ref == 'main' / if: github.base_ref != 'main' guards instead, each with an unambiguous literal commitDepth. The job name (Lint commit messages) is unchanged, so this stays the same required check.

Scope

Touches only .github/workflows/commitlint.yml. No source changes, no other workflow.

Test plan

• CI run on this PR (main-base) exercises the new commitDepth: 1 step and the fixed title-suffix lint against its own commit/title — must pass, proving the fix isn't self-blocking.
• A future develop -> main cut PR should no longer be blocked by inherited, already-linted commits.
• A future PR whose title is within 100 chars but whose title + (#NNN) suffix exceeds it should now be caught before merge.

🤖 Generated with Claude Code

[MichaelTaylor3d]

This PR has been superseded by #602. The commitlint workflow fix was cherry-picked onto the release/0.256.0 branch, which already carries the v0.256.0 version bump. Folding the workflow change into #602 creates a single PR that satisfies both the commitlint check (fixed workflow) and the version increment gate (v0.256.0 bump).

The ci/scope-commitlint branch is preserved for reference if the fold needs to be re-examined.