chore: FXC-4066-improve-changelog-management

[marcorudolphflex]

Summary

This PR migrates release-note management from manual CHANGELOG.md edits to Towncrier fragments and adds automation to generate changelog PRs for releases.

Changes

• Added Towncrier support to the project:
  • towncrier in dev dependencies (pyproject.toml, poetry.lock)
  • [tool.towncrier] config in pyproject.toml
  • new template at changelog.d/template.md
  • contributor guidance at changelog.d/README.md
• Migrated current unreleased entries into fragment files under changelog.d/*.md.
• Updated CHANGELOG.md to Towncrier-managed format with start marker:
  • <!-- towncrier release notes start -->
• Added scripts/changelog_refs.py to update release compare links automatically.
• Added workflow public/tidy3d/python-client-build-changelog-pr to:
  • build CHANGELOG.md from fragments
  • update compare links
  • open an automated PR
• Updated CI branch-name lint rule to exempt auto-generated changelog branches:
  • chore/build-changelog-*
• Updated docs and contributor guidance:
  • .github/workflows/README.md
  • docs/development/release/flow.rst
  • docs/development/release/version.rst
  • AGENTS.md

Demo

Here is one demo run of the github action on a fork:
marcorudolphflex#4

---

Note

Medium Risk
Moderate risk due to new CI gating and release automation that can block merges or generate incorrect changelog/compare links if the workflow or reference updater misbehaves.

Overview
Migrates release notes from direct CHANGELOG.md edits to Towncrier fragments in changelog.d/, including a Towncrier template/config, contributor docs, and moving current unreleased entries into fragment files; CHANGELOG.md is reformatted as a generated file with a Towncrier marker.

Adds automation for release managers: a new public/tidy3d/python-client-build-changelog-pr workflow that builds the changelog from fragments, updates compare links via new scripts/changelog_refs.py (with tests), and opens an auto PR. CI is updated to block PRs that edit CHANGELOG.md directly (except auto-generated chore/build-changelog-* branches) and to exempt those branches from Jira-key branch-name enforcement; docs/agent guidelines and dev deps (towncrier) are updated accordingly.

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[github-actions]

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[momchil-flex]

Seems neat!

[yaugenst-flex]

Thanks for setting up Towncrier-based changelog generation and automation. I found one actionable edge case in scripts/changelog_refs.py: if a release version is provided with a leading v, the script currently generates malformed compare links (...vvX.Y.Z). Please normalize new_version to strip a leading v before building reference links, and add a small regression check for this CLI input form.

[daquinteroflex]

Looks good thanks!

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