Add repo-specific RELEASE.md (tag is a branch here, not a tag)

[jnasbyupgrade]

Summary

Investigated whether trunklet needs a local RELEASE.md now that the org
has a shared one (Postgres-Extensions/ai#8, not yet merged). It does, for
genuine repo-specific reasons — this repo's actual release mechanics
diverge from the shared doc in ways that would produce wrong results if
followed verbatim:

• make tag creates a git branch, not a tag. This repo's vendored
  pgxntool/base.mk (squashed in via git subtree, not the submodule
  other repos use — a much older vintage) implements tag: as
  git branch $(PGXNVERSION); git push --set-upstream origin $(PGXNVERSION).
  Verified: git tag -l and git ls-remote --tags are both empty, while
  every past release (0.0.1 .. 0.3.3) exists as a branch on origin.
  The shared doc's own "Notes / gotchas" section already anticipates this
  exact situation ("older pgxntool/README.asc wording describing the
  result as a 'branch'") — trunklet's vendored README.asc still says
  this, and unlike some repos it's not just stale wording, the Makefile
  genuinely still does it.
• No stable pseudo-version. default_version/META.in.json version
  are always a real released number (currently 0.3.3), never the literal
  string stable — this vintage predates that mechanism entirely.
• No verify-results target. Only results (regenerate expected
  output) and installcheck (marked .IGNORE) exist, so a green
  make test doesn't prove pg_regress passed.

This PR adds a minimal RELEASE.md documenting only these three concrete
deviations and pointing to ../ai/RELEASE.md for everything else.

Depends on Postgres-Extensions/ai#8 landing first for the linked doc to
resolve (https://github.com/Postgres-Extensions/ai/pull/8).

Test plan

• Verified git tag -l / git ls-remote --tags upstream are empty and
  all nine version branches (0.0.1..0.3.3) plus variant are
  ancestors of master (confirming they're historical branch-based
  "tags", not divergent release lines).
• Confirmed master HEAD == branch 0.3.3 HEAD exactly (same SHA).
• Read vendored pgxntool/base.mk's tag/rmtag/dist targets and
  pgxntool/README.asc's description of tag.
• Confirmed no stable version anywhere in trunklet.control /
  META.in.json, and no verify-results target in base.mk.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on this repository. Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: b78920d4-5887-43ad-af46-686e5369107b

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}