Draft GitHub releases don't create git tags, and force-tag-creation
requires release-please >= 17.2.0 (action v4.4.0 bundles 17.1.3).
Without the tag, RP falls back to scanning all commits and computes
a bogus version (e.g. 0.2.0 instead of 0.2.7).

Add a pre-RP step that reads the manifest version and creates the
tag if missing. Remove once googleapis/release-please-action#1187
ships.

Also: created v0.2.6 tag manually and closed bogus PR #471.

## Summary

- Add a pre-RP step to `release.yml` that reads the manifest version and
creates the git tag if missing — draft GitHub releases don't create
tags, and `force-tag-creation` requires RP >= 17.2.0 (action v4.4.0
bundles 17.1.3)
- Update CLAUDE.md to document the workaround

## Context

Every release produced a bogus `release 0.2.0` PR because RP couldn't
find the latest release tag. Root cause: `force-tag-creation` flag is
silently ignored in RP 17.1.3 (feature added in 17.2.0). Waiting on
[googleapis/release-please-action#1187](googleapis/release-please-action#1187).

Also done outside this PR: created `v0.2.6` tag manually, closed bogus
PR #471.

## Test plan

- [ ] Next release PR merge creates the tag before RP runs
- [ ] RP finds the tag and does not produce a bogus version PR
- [ ] Remove workaround step once action v4.4.1 ships