docs(release): tags become vYYYY.MM.DD.HHMM, and stop telling people to move them

The tag is now the artifact's own version name with a `v` in front, so
`v2026.09.10.1432` and `2026.09.10.1432` are one string. Nothing has to
reconcile what the tag claims against what the APK reports, and minting one
is arithmetic on the tagged commit's timestamp rather than a lookup.

The substantive change is the prose. release.yml's header instructed the
reader to `git push -f origin vYYYY.MM.DD` on a same-day re-cut. That is
the operation the family rulebook forbids outright, and it has incidents
behind it — moving a same-day tag forward once took a published release
down with it. Anyone who had installed from that tag was holding something
it no longer pointed at.

With HHMM there is nothing left for mutability to buy: every tag is unique
by construction, so a second release the same day is not a collision to
resolve, just another tag.

The old instruction is recorded as retired rather than deleted. Someone who
remembers it should learn it was withdrawn and why, not find it silently
absent and assume they misremembered.

README contradicted itself inside one sentence — "immutable per-day release
tags ... a same-day re-cut moves the tag forward" — and now says which it
is, plus a note that pre-2026-09-10 tags keep the old shape and still work.

Transition wrinkle, deliberately left for step 3: the non-tag :latest path
reconstructs the bundled APK's name from the latest release's commit
timestamp, which for the one existing old-shape release yields
2026.09.09.1828 while that APK actually declares 2026.09.09.1895. It fails
SAFE — 1828 compares lower, so no false update is offered — and it
self-corrects at the first new-scheme release. Step 3 removes the
reconstruction entirely by having the sidecar carry recorded values instead
of derived ones.

Step 2 of 5 — Scribe task #3809, milestone #390.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

README.md

@@ -113,10 +113,12 @@ Most operational keys have a `MINSTREL_<SECTION>_<FIELD>` env override. Recommen
 Image tags (`git.fabledsword.com/bvandeusen/minstrel:<tag>`):
 
 - `:latest` — the newest blessed image. Moves on every `main` push **and** every release. Recommended for most operators.
-- `:vYYYY.MM.DD` — immutable per-day release tags. Pin one of these for a deployment you don't want moving under you. (Per-day CalVer — no trailing patch digit; a same-day re-cut moves the tag forward.)
+- `:vYYYY.MM.DD.HHMM` — immutable release tags, never moved or deleted. Pin one for a deployment you don't want changing under you. The tag is the build's own version name with a `v` in front, derived from the tagged commit's UTC timestamp, so two releases can never collide and a re-cut is simply a new tag.
 - `:main` — the rolling post-merge tip. Same image as `:latest` at push time; choose it if you want to track `main` explicitly rather than the release line.
 
-Every `:latest` and every `:vYYYY.MM.DD` bundles the current signed Android APK, so the in-app update channel is always live. Database migrations run automatically at startup; rollbacks require restoring a Postgres dump.
+Every `:latest` and every `:vYYYY.MM.DD.HHMM` bundles the current signed Android APK, so the in-app update channel is always live. Database migrations run automatically at startup; rollbacks require restoring a Postgres dump.
+
+Releases before 2026-09-10 use the older per-day `:vYYYY.MM.DD` shape. Those tags still exist and still work — they are simply not extended.
 
 ## Specs