chore: release 2.1.1

[micaelaw]

Summary

• Empty commit with Release-As: 2.1.1 footer to override release-please's bump calculation
• Targets shipping the fix: from fix: remove unused python-dotenv dependency #63 (python-dotenv removal) as 2.1.1, superseding PR chore(main): release 2.0.6 #64 (which incorrectly proposes 2.0.6)

Why this is needed

The manual 2.1.0 bump in #61 only edited in-repo version strings — it never produced a git tag, GitHub release, or PyPI publish (CD only fires when release-please itself sets release_created=true). From release-please's perspective the last released version is still v2.0.5, so its math on the fix: in #63 yielded 2.0.5 + fix → 2.0.6, rolling back the in-repo version. The Release-As footer is release-please's official override for exactly this situation.

Expected sequence after merge

1. CD runs release-please on main; it regenerates/supersedes chore(main): release 2.0.6 #64 as chore(main): release 2.1.1.
2. Merging that release-please PR creates tag v2.1.1, the GitHub release, and triggers poetry publish to PyPI.
3. PyPI consumers go directly from 2.0.5 → 2.1.1 (2.1.0 was never published, so no regression).

Test plan

• After merge, CD workflow run on main succeeds
• release-please PR appears or chore(main): release 2.0.6 #64 updates to title chore(main): release 2.1.1, with all five tracked files bumped to 2.1.1 and fix: remove unused python-dotenv dependency #63's fix in the changelog
• Merging that release-please PR creates tag v2.1.1 and a v2.1.1 GitHub release
• pip install shipengine==2.1.1 succeeds in a clean venv

Follow-up (separate PR, not blocking)

Add a Conventional Commits PR-title linter (e.g. amannn/action-semantic-pull-request) so squash-merge titles can't drop feat:/fix: prefixes again — that's the proximate cause of this whole detour (PR #58's feat: was lost on squash-merge, forcing the manual #61 workaround).

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