fix: publish beangulp-yapeal + close two core bugs PR #1 exposes

[borgoat]

Depends on #1

This branch is built on top of jensbin's yapeal branch content (it's not
mergeable to main until #1 lands — the diff below will shrink to just
this PR's commit once that happens).

Summary

Follow-up from reviewing #1. Splitting the fix so the CI gap and the
core-quints bugs get fixed by us, and the yapeal-package-specific issues
go back to the PR author (see review comments on #1):

• CI: .github/workflows/publish.yaml's publish matrix never listed
  beangulp-yapeal, even though quints now hard-depends on it at
  runtime. The next version tag would publish quints with an
  unresolvable dependency (uvx quints fails to resolve).
• init.py: quints init --importers yapeal never opened the yapeal
  bank account in accounts.bean. Reproduced end-to-end — booking a
  drafted statement and running quints check fails with "Invalid
  reference to unknown account". Fixed _backbone() to open it, same as
  the wise/stripe branches.
• importing.py: match_receivables() only rewrote the counter-leg
  account when a draft already had two postings. Yapeal's default
  (no-rule-matched) draft has only the cash leg, so a receivable match
  flipped it to flag * without completing it — beancount rejects the
  resulting single-posting transaction as unbalanced on load. This is a
  pre-existing latent bug in shared code, newly reachable via yapeal's new
  caller; fixed to append the elided receivable posting so the match
  actually balances.
• Mechanical drift: stale "supported: ubs, wise, stripe" strings in
  init.py, the --importers CLI help text, and docs/getting-started.md;
  missing yapeal reminder in the generated AGENTS.md sample-data
  checklist.

Note: adding beangulp-yapeal to the publish matrix also needs a
pypi-beangulp-yapeal GitHub environment (mirroring the existing
pypi-<package> ones) and the matching PyPI trusted-publisher
registration — that's a one-time manual setup step, not something a code
change can do.

Test plan

• Added a regression test that scaffolds a yapeal-configured project
  and loads the ledger end-to-end (would have failed before the fix).
• Added a regression test for match_receivables() completing a
  single-posting draft, including a full load through
  beancount.loader.load_string.
• make check — green (ruff, basedpyright, import-linter, deptry,
  vulture, 186 passed / 2 skipped).

🤖 Generated with Claude Code