Post-release workflow: immediately bump default_version to 'unstable'

[jnasbyupgrade]

Problem

Now that we recommend committing versioned SQL files (e.g., sql/myext--1.2.0.sql), there's a window after a release where developers can accidentally commit changes to the released version file. If you release 1.2.0 and then keep working without bumping the version, your next make regenerates sql/myext--1.2.0.sql with new changes that weren't part of the 1.2.0 release.

Previously this wasn't an issue because versioned files weren't checked in — only upgrade files were. But with versioned files in git, the released version file must be frozen immediately after release.

Proposed solution

After tagging a release, the release process should immediately update default_version in the .control file to unstable. This:

1. Makes it obvious you're on a development version
2. Causes make to generate sql/myext--unstable.sql instead of overwriting the released file
3. The unstable versioned file should be added to .gitignore (via _.gitignore) so it never gets committed

When you're ready for the next release, you update default_version to the actual new version number.

Note: unstable breaks semver, but PostgreSQL doesn't mandate semver for extension versions, so this is fine.

Changes needed

• Update _.gitignore template to ignore sql/*--unstable.sql
• Update release process/skill to bump default_version to unstable after tagging
• Document the workflow in README