chore: adopt the AGENTS.md convention (add CLAUDE.md symlink)

[kw-tmueller]

Adopts the AGENTS.md convention, which specifies the filename as uppercase AGENTS.md — the casing agent tooling looks for, and the only one found on the case-sensitive filesystems of our Linux CI runners.

• agents.md → AGENTS.md (content unchanged apart from the H1 filename)
• CLAUDE.md added as a symlink to AGENTS.md, so Claude Code reads the same file with no second copy to keep in sync

Opened by scripts/sync-agents-md.py in owncloud/admin.

On Windows clones without core.symlinks=true, CLAUDE.md materialises as a one-line text file containing AGENTS.md. AGENTS.md itself is always correct.

[CLAassistant]

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