Add rules for coding agents [skip ci]

The rules sco8487 and hunterk proposed in the testers' Discord thread:
the main and libretro branches are never deleted or rewritten, features
go on their own branch and are squashed into libretro after testing,
only the developer edits these files, and the code-side lessons from
testing - keep comments true, one path for reset and close content,
upstream's way of stopping threads, no core options that do nothing.
CLAUDE.md only imports AGENTS.md, so there is one copy of the rules.

AGENTS.md

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+# Rules for coding agents
+
+These apply to any AI agent working in this repository (Claude Code reads them through `CLAUDE.md`). They came out of testing the libretro core with its testers.
+
+## Branches and history
+
+- The `main` and `libretro` branches are never deleted, and nothing that rewrites their history is run on them: no force-push, no rebase, no amending or squashing of commits that are already pushed.
+- New features and experimental changes go on their own branch (for example `audio-stall`), and testers test builds of that branch. When testing is done, the related commits are squashed and merged into `libretro`, so the main branch does not collect commits that were superseded midway.
+- A fix that users of the `libretro` builds need before the branch is merged is cherry-picked into `libretro`, in a way that does not break the later merge of the branch.
+
+## These files
+
+- Only the developer writes to or deletes `AGENTS.md` and `CLAUDE.md`. An agent does not change them on its own; it proposes the change to the developer instead.
+
+## Code
+
+- Keep comments true. When a change makes a comment describe behaviour that no longer exists, fix or remove the comment in the same commit.
+- Look at the big picture, not just the function being changed. For example, resetting the content and closing it have to release the game's resources through the same path; a reset is an unload and a load.
+- Stop and join threads the way upstream Cemu does, not with methods made up for the libretro port. The ad-hoc ones are what caused most of the shutdown and reset bugs.
+- Every core option has to be connected to something. Do not add an option the core does not read, and remove one that turns out to do nothing.
+
+## Testing
+
+- `log.txt` names the commit it was built from, under "Loaded title". Ask testers for `log.txt` and the RetroArch log, and check which commit a log came from before drawing conclusions from it.

CLAUDE.md

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+@AGENTS.md