Install guide: use a tested release tag instead of dev #106

Both install guides tell users to clone dev, which changes on every commit. A user who follows the guide can get a broken build, as happened on 2026-10-20.

This issue changes no code. Do it first. It does not need #107 or #108.

Where users are sent to dev

On 2026-10-20 an outside user followed sitmun.org → Projecte → sitmun.github.io → Instalación. docker compose up -d built no containers, because #97 had just broken the front image (fixed by #99, prevented by #108).

Facts

• main is the default branch and equals the sitmun-application-stack/1.2.9 tag (a2777001, 2026-09-20). Only --branch dev puts users on unreleased code.
• sitmun-application-stack/1.2.9 has not been verified to start with docker compose up -d on a clean clone.
• This repository has tags but no GitHub Releases (gh release list is empty). Release notes exist only in CHANGELOG.md.
• The guide says ./setup.sh copies .env.example. That file was removed (CHANGELOG.md). setup.sh copies profiles/development-postgres.env when .env is missing.
• 1.3.0, the first LTR (LTR 1.3.0 #104), is about to be released and is the natural tag to name.

Tasks

• On a clean machine, clone the target tag (1.3.0 when cut, or 1.2.9 in the meantime) with --recurse-submodules, run ./setup.sh and docker compose up -d, and confirm admin and viewer load. Record the result on the release.
• Publish a GitHub Release for that tag, with notes from its CHANGELOG.md section.
• Change the end-user steps in both guides to git clone --branch sitmun-application-stack/X.Y.Z --recurse-submodules ….
• Move --branch dev into a separate "Development" section that says dev can be broken.
• Replace .env.example in the guide with profiles/development-postgres.env.
• Add making a GitHub Release, and updating the tag in both guides, to RELEASE.md.

Acceptance

• Following the public guide with no extra knowledge installs a tagged release that has been verified on a clean clone.
• That tag has a GitHub Release with notes.

Related: #107, #108.