Add AGENTS.md

This helps steer LLMs when operating on this codebase - https://agents.md/

Mostly to help me with making releases for now

AGENTS.md

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+## Building and Testing
+
+- To compile the project: `meson compile -C build` (compiles everything, no target needed)
+- To run tests: `meson test -C build`
+- The build artifacts always live in a directory called `build` or `builddir`.
+  If `build` doesn't exist, use `-C builddir` in the meson commands.
+
+## Git Workflow
+
+- Main branch: `master`
+- Always create feature branches for new work
+- Use descriptive commit messages following project conventions
+- Reference GitLab MR/issue numbers in commits where applicable
+- Never commit build artifacts or temporary files
+- Use `glab` CLI tool for GitLab interactions (MRs, issues, etc.)
+
+## Making a release
+
+- Each release always consists of an entry in NEWS.rst, at the top of the file, which describes
+  the changes between the previous release and the current one. In addition, each release is given
+  a unique version number, which is present:
+    1. on the section header of that NEWS.rst entry
+    2. in the project() command in meson.build
+    3. on the commit message of the commit that introduces the above 2 changes
+    4. on the git tag that marks the above commit
+- In order to make a release:
+    - Begin by analyzing the git history and the merged MRs from GitLab between the previous release
+      and today. GitLab MRs that are relevant always have the new release's version number set as a
+      "milestone"
+    - Create a new entry in NEWS.rst describing the changes, in a similar style and format as the
+      previous entries. Consolidate the changes to larger work items and also reference the relevant
+      gitlab MR that corresponds to each change and/or the gitlab issues that were addressed by each
+      change.
+    - Make sure to move the "Past releases" section header up, so that the only 2 top-level sections
+      are the new release section and the "Past releases" section.
+    - Edit meson.build to change the project version to the new release number
+    - Do not commit anything to git. Let the user review the changes and commit manually.