Greptile Summary

This PR simplifies agent-instruction housekeeping: CLAUDE.md becomes a symlink to AGENTS.md so Claude Code and Codex share one canonical file, the two stale GSoC redirect files are removed, and the test-convention section in CONTRIBUTING.md is expanded with clearer guidance.

• CLAUDE.md is replaced by a symlink to AGENTS.md; the old single-line redirect is gone.
• GSOC_2026_PROPOSALS.md and GSOC_CONTRIBUTOR_GUIDANCE.md are deleted; all GSoC references are stripped from AGENTS_EXTERNAL.md as well.
• CONTRIBUTING.md test conventions gain new items: monkeypatch for env vars/attributes, a preference for pure-function tests over mocks, explicit ids= on @pytest.mark.parametrize, and a generalised test/<area>/ path pattern.

Confidence Score: 5/5

Documentation-only change; no runtime code is touched and the symlink approach is straightforward.

All changes are to markdown files and a symlink. The symlink correctly points CLAUDE.md to AGENTS.md, the deleted GSoC files were thin redirect stubs, and the CONTRIBUTING.md additions are additive improvements with no regressions possible.

AGENTS_EXTERNAL.md — the self-propagation rule for non-symlinked agent config formats (e.g. .cursorrules) was removed; worth confirming this is intentional.

Important Files Changed

_{Reviews (1): Last reviewed commit: "Simplify agent and test guidance" | Re-trigger Greptile}