docs: add AGENTS.md agent guidelines + CLAUDE.md pointer

[willhea]

Summary

Add AGENTS.md (shared agent/contributor guidelines) and a one-line CLAUDE.md pointer to it. No agent-instruction file existed before.

Why

AGENTS.md captures the non-obvious conventions and gotchas an agent or new contributor needs, without restating the README:

• PRs target develop, not main.
• The npm audit production gate blocks merges; fix via targeted bump / overrides, never audit fix --force.
• Color comes from brand CSS-variable tokens, not raw Tailwind palette; diff-op colors in one DIFF_OP_STYLE map.
• Dark mode is non-functional / being removed (don't add dark:).
• Diff-engine split (DeltaTrack Python engine ↔ TS fallback), two diff renderers, the ESLint flat-config quirk, MySQL/knex, next-auth v5.

Notes

• Committed as shared project docs; personal/local Claude state stays in the already-gitignored .claude/.
• Lean by design — links to the README for spin-up and the full command list.
• One forward reference: the src/lib/diff-op-styles.ts mention assumes the diff-op token-unification PR has landed.