Add AGENTS.md for cross-session agent context

Covers what a fresh session needs that statuspage/README.md doesn't:
the Windows-checkout-vs-FreeBSD-runtime split (nothing here actually
executes the check scripts' real commands), the git push/pull
deployment loop, the core.fileMode/pre-commit-hook setup, and five
gotchas hit and fixed this session (usec/sec substring match, ntpq's
"+" prefix, grep -c's exit-1-on-zero, Windows ln -s on directories
falling back to a copy, font glyph coverage).

CLAUDE.md is a pointer to AGENTS.md rather than a symlink or a
duplicate copy - this checkout can't reliably create real symlinks
(same root cause noted in AGENTS.md itself), and a stale copy would
silently drift out of sync as AGENTS.md changes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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