Update AGENTS guidance

[Silex]

This updates AGENTS.md after the earlier rename from CODEBUDDY.md.

The renamed file still identified itself as CODEBUDDY.md and described itself as guidance for CodeBuddy Code specifically. That mismatch is confusing for both humans and agents because the repository now points backends at AGENTS.md, but the file content still suggested a backend-specific instruction file. The practical effect is that the primary agent guidance file looked like a copied rename rather than an intentional project-level document.

The same file also carried stale command examples in the testing section. The single-test example omitted the current test/ path prefix, and the "run all tests" example hard-coded an older subset of files. Those examples were no longer aligned with the repository layout, so following them would fail or miss large parts of the suite.

This change makes the file consistent with its new role by renaming the heading to AGENTS.md, broadening the opening description to coding agents in general, and replacing the test examples with commands that match the current tree. The updated commands add -L . so the project files are on load-path, keep the single-file example pointed at an existing test file, and switch the full-suite example to a wildcard-based loader so it is less likely to drift as the suite evolves.

Validation was limited to command-level sanity checks in the current environment. I verified the document diff directly, confirmed that the wildcard-based command loads the current test/test_*.el files once -L . is added, and confirmed that the remaining failures are environmental rather than caused by the documentation update: this environment is missing at least magit, and the available transient is older than what test/test_ai-code.el requires.