docs(release): refresh releasing.md wording that predates the 0.1.0 publish

[tbolis-at-mulesoft]

Closes #687

Summary

• donkey-kit shipped 0.1.0 to PyPI on 2026-09-25 (Publish donkey-kit to PyPI and wire project URLs #339); three passages in
  docs/releasing.md still described the project as unpublished. Reworded:
  the .devN ladder bullet (dropped the stale "Today's main is here"
  marker and the "no usable feature surface" framing), the one-time trusted
  publisher setup paragraph (now points at editing the existing publisher,
  keeping the pending-publisher form only as the not-yet-exists fallback),
  and the closing irreversibility warning (no more "pending production
  publisher"; cites Publish donkey-kit to PyPI and wire project URLs #339 as completed).
• Also generalized the pre-release ladder illustration (ASCII diagram + the
  .devN/final bullets) from the concrete 0.1.0 to the file's own X.Y.Z
  placeholder idiom (already used elsewhere in the same doc), so it can't go
  stale again at the next final release. The milestone-table example row and
  the PEP 440 spelling/tag examples (0.1.0a1, v0.1.0a1) stay concrete —
  they illustrate the naming form, not current state.

Test plan

• pytest -q — 696 passed, 47 skipped, 6 deselected
• mypy — no issues found in 59 source files
• ruff check . — all checks passed
• lint-imports — 5 contracts kept, 0 broken
• rg verification that no stale phrasing (not yet published, pending publisher as the primary path, No usable feature surface, Today's main is here, pre-MVP groundwork) or stray 0.1.0 ladder literal
  remains in docs/releasing.md

Post-deploy steps

None.