chore: adopt the AGENTS.md convention (fold CLAUDE.md into AGENTS.md)

[kw-tmueller]

Brings this repo onto the AGENTS.md convention. Refs owncloud/admin#249.

AGENTS.md    100644   renamed from agents.md
CLAUDE.md    120000   symlink -> AGENTS.md

• The spec says uppercase AGENTS.md, and that is what agent tooling looks for — a lowercase agents.md is simply not found on the case-sensitive filesystems of our Linux CI runners.
• CLAUDE.md becomes a symlink to AGENTS.md, so Claude Code reads the same content and there is no second file to keep in sync.

This one was done by hand. The org-wide sweep (scripts/sync-agents-md.py in owncloud/admin) deliberately refuses any repo whose CLAUDE.md is a real file, because replacing it with a symlink would silently discard its content. Here that content has been folded into AGENTS.md first, so nothing is lost:

• The "Website changes require a screenshot" rule is carried over verbatim as its own section.
• Also fixed while here: the .nvmrc note said Node 20, but .nvmrc actually pins 22.12.0 (CLAUDE.md had it right).
• REUSE.toml: the explicit path array listed "agents.md"; it now lists "AGENTS.md" and "CLAUDE.md", so reuse lint still passes.

Guidance that CLAUDE.md merely repeated from agents.md (OSPO policy, signed commits, DCO sign-off, Conventional Commits) was not duplicated.

One signed commit. Verify the symlink with git ls-tree HEAD — CLAUDE.md should be mode 120000 with a 9-byte blob.

Windows caveat: without core.symlinks=true plus Developer Mode, CLAUDE.md materialises as a one-line text file containing AGENTS.md. AGENTS.md itself is always correct, so nothing breaks.

🤖 Generated with Claude Code