docs: adopt semantic versioning — release 1.27.1 tracks Gitea 1.27.1

Templates were re-checked against the local gitea clone (v1.27.1 tag): all
46 translation keys exist in locale_en-US, all data paths match the official
templates, and all 110 templates render via `go run . preview all` — no
template changes required.

From now on release versions mirror the Gitea version they support:
- COMPATIBILITY.md: quick reference now leads with release 1.27.1
  (max tested Gitea 1.27.1, active), v1.0.x marked superseded; new
  Versioning section documents the scheme
- README: release version in header + versioning bullet
- Tracker workflow PR body: added semantic-versioning release checklist
  (bump Template Release column, tag vX.Y.Z)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -4,7 +4,7 @@ A curated collection of professionally designed, audience-driven email templates
 
 [![Gitea](https://img.shields.io/badge/Gitea-1.25+%20%7C%201.27.1%20tested-blue)](COMPATIBILITY.md) <!-- TRACKER:BADGE -->
 
-> **110 template files — 10 visual styles, 11 email types each**
+> **Release 1.27.1 — 110 template files, 10 visual styles, 11 email types each**
 
 ---
 
@@ -165,6 +165,7 @@ gitea-mail-templates/
 
 - **Gitea 1.25+** — matches the refactored mail template directory structure (v1.25)
 - **Latest tested:** Gitea 1.27.1 <!-- TRACKER:LATEST-TESTED -->
+- **Versioning:** releases mirror the Gitea version they support — Gitea 1.27.1 → template release 1.27.1 (see [COMPATIBILITY.md](COMPATIBILITY.md))
 - 100% variable-compatible with official Gitea templates — see [COMPATIBILITY.md](COMPATIBILITY.md) for the full matrix
 - Uses only built-in Gitea template functions and official translation keys
 - No custom template functions or locale patches required