Untyped changelog fragments are silently left out of CHANGELOG.md #2314

Towncrier only consumes fragments named <name>.<type>.md, with a type configured under [[tool.towncrier.type]] in pyproject.toml (breaking, added, changed, fixed, removed). Any other file in changelog.d/ is never written to CHANGELOG.md and never deleted, and nothing tells the author.

Checked on main at 6d89338 (2026-10-11):

• changelog.d/ holds 93 files without a type, such as 1005.md and 1044-explicit-leave.md. None of their notes is in CHANGELOG.md.
• .github/bump_version.py counts every file except .gitkeep, so these files make every versioning run bump at least a patch. When a push carries only untyped files, towncrier writes No significant changes.; CHANGELOG.md has 73 such releases.
• Since 2026-07-01, 134 merges to main triggered versioning. Five carried only untyped fragments (Add person-level bus fare allocation #1801, Fix the capital gains realisation response #1803, Correct the capital gains rate effective date and annualise the split year #1806, Fix hours_worked units in participation dynamics #1828, Uprate electricity_consumption and gas_consumption #1860), so their notes were lost. None of the 93 files was added alongside a typed fragment.
• Among 126 open PRs, one adds an untyped fragment (Reconcile UC deduction and benefit cap components under the protected floor #1819).

Also, towncrier silently drops an empty fragment. A fragment that starts with - renders as - - because the default template adds the bullet; docs/engineering/skills/github-prs.md shows that form in its example (#2162 item 3).

Fix:

1. A pull-request check that fails when a PR adds or modifies a file in changelog.d/ that towncrier would not consume as <slug>.<type>.md with a configured type, or an empty fragment.
2. Delete the 93 stale untyped files, after checking that each one's PR is merged and released.
3. bump_version.py counts only the fragments towncrier consumes, and warns about the rest.