chore(conventions): switch product-repo tag form to bare SemVer numerals

[larsweiser]

Summary

Aligns docs/PRODUCT_REPO_CONVENTIONS.md with the bare-numeral SemVer
tag form per the SemVer 2.0.0 FAQ:

v1.2.3 is not a semantic version. […] the canonical form has no
v prefix.

Mirrors thalura-plugin#5
(already merged) which applied the same change to thalura-plugin's
release.yml. thalura-plugin shipped 0.1.0 on 2026-05-13; the
conventions doc was the only canonical artefact still mandating the
legacy vX.Y.Z form.

What changed

Docs-only — single file, docs/PRODUCT_REPO_CONVENTIONS.md (+33/-29).

• §2 — tag-format bullets rewritten to X.Y.Z with SemVer FAQ link
• §3.1 — CHANGELOG heading example + paragraph (canonical named first)
• §3.2 — "rename to" instruction updated
• §4 — YAML top-comment + trigger glob ('v*' → '[0-9]+.[0-9]+.[0-9]+*')
• §4 — "notes on workflow" bullet (canonical named first)
• §7.1 / §7.2 — UI + CLI release procedures
• §8.1 / §8.2 / §8.4 / §8.5 — failure-mode examples; §8.5 inverted

Explicitly preserved

• awk extractor body + error message at lines 145–158 — keeps the
  two-form tolerance (\\[v"tag"\\]) for backwards compatibility
  with legacy ## [v0.1.0] headings.
• actions/checkout@v6 reference — action-version pin, not a
  tag-form example.

Website pipeline impact

None. The release loader is tag-form-agnostic end-to-end — tagName
flows verbatim from GitHub REST → src/lib/github-api.ts:120 →
src/content.config.ts:186 → src/pages/produkte/[slug].astro:124
(version-chip). No v-prefix manipulation anywhere in src/ or
scripts/. Both 0.1.0 and v0.1.0 round-trip identically.

Verification

grep -c 'vX\.Y\.Z' docs/PRODUCT_REPO_CONVENTIONS.md   # 1 (awk comment, preserved)
grep -c "'v\*'" docs/PRODUCT_REPO_CONVENTIONS.md       # 0
grep -c "'\[0-9\]+\.\[0-9\]+\.\[0-9\]+\*'" docs/PRODUCT_REPO_CONVENTIONS.md  # 1
grep -c 'semver.org/spec/v2.0.0.html' docs/PRODUCT_REPO_CONVENTIONS.md      # 2
grep -c 'seen=1; next' docs/PRODUCT_REPO_CONVENTIONS.md                     # 1 (awk untouched)

Test plan

• Visual review of the §-by-§ diff in this PR.
• After merge to dev + rollout to main: pipeline-test-fixture
  will be updated in the GitHub UI to mirror this convention as
  the live end-to-end integration test (Lars-driven).

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