ci: fix release pipeline and package metadata for PyPI publishing

[legion-implementer]

Why

We now own https://pypi.org/project/pivot/ 🎉 — but the Release workflow has never succeeded (every run since February failed), so no version was ever bumped or published. This PR fixes every bug blocking the first automated release.

Bugs fixed

1. Detached HEAD (the actual CI failure) — actions/checkout pins head_sha, leaving HEAD detached; python-semantic-release refuses to release outside a release branch (Detached HEAD state cannot match any release groups). Fixed with git checkout -B main before running semantic-release.
2. Version bumps never reached the packages — version_toml only bumped the root workspace pyproject.toml; packages/pivot and packages/pivot-tui would have published 0.1.0 forever. All three are now bumped in lock-step.
3. Nothing publishable was built — bare uv build at the workspace root doesn't build workspace members. Now uv build --all-packages.
4. Release-commit guard mismatch — the workflow skips commits starting with chore(release):, but PSR's default commit message doesn't match, so release commits would pointlessly re-run the pipeline. commit_message now aligned.
5. Pre-alpha versioning — major_on_zero = false / allow_zero_version = true keeps us in 0.x even if BREAKING CHANGE: commits land.
6. Unpublishable metadata — license = TBD / authors = Internal Team replaced with MIT + real author; added LICENSE files, per-package READMEs (PyPI long description), [project.urls], and classifiers to both packages. pivot-tui had no license/author/readme metadata at all.

Verification

• semantic-release --noop version --print on a clone of this branch computes 0.1.0 (matches current pyproject version; PyPI already has 0.0.1/0.0.2 from the previous owner, so this is a valid next version)
• uv build --all-packages → 4 artifacts; twine check dist/* → all PASSED; wheel METADATA shows License-Expression: MIT, bundled LICENSE, author, URLs
• ruff format --check, ruff check, basedpyright clean; full test suite passes (3570 + 428 tests)

After merge

Merging this (contains no feat:/fix:, but the entire unreleased history counts) triggers Tests → Release → semantic-release tags v0.1.0, creates the GitHub release, and uv publish uploads pivot and pivot-tui to PyPI via trusted publishing.

⚠️ Requires PyPI-side setup before merge (@sjawhar):

• pivot: Manage → Publishing → add GitHub publisher: owner sjawhar, repo pivot, workflow release.yaml, environment pypi
• pivot-tui: account → Publishing → add pending publisher with the same settings (first upload creates the project)

Note: from this point on, every merge to main with feat:/fix: commits auto-publishes a release.