docs: jsfeat parity/refactor audit plan + cross-agent instructions

[kalwalt]

What & why

Adds a durable planning artifact comparing jsfeatNext against the original inspirit/jsfeat, plus cross-agent instruction files. No library code changes.

Produced via a structured /brainstorming session; tracked by #46.

Added

• docs/jsfeat-parity-and-refactor-audit.md — the plan, organized in 5 axes + appendices:
  • Axis 1 — function-and-parameter-level feature parity (incl. missing haar / bbf)
  • Axis 2 — API/behavioral parity (jsfeatNext.jsfeatNext double namespace, static-vs-instance methods, per-instance cache)
  • Axis 3 — clean-code pitfalls (type-only stub files duplicating the ~3,900-line jsfeatNext.ts monolith)
  • Axis 4 — recommended target architecture + phased, test-gated migration
  • Axis 5 — build modernization (webpack → Vite) + dependency cleanup
  • Appendices — haar/bbf porting plan; Vitest characterization-test harness (Phase 0 gate)
• AGENTS.md — canonical, tool-agnostic agent guidance (build commands, architecture, the stub/monolith gotchas)
• CLAUDE.md — thin; @AGENTS.md import + Claude-specific notes
• .agents/instructions.md, .github/copilot-instructions.md — pointers to AGENTS.md
• .gitignore — ignore .idea/

Reviewer notes

• This is a planning/analysis document, not implementation — the function-level parity tables and clean-code passes are scoped as follow-up work (issues Test harness: Vitest characterization tests vs jsfeat (Phase 0 gate) #39–Refactor: de-duplicate type-only stubs and the jsfeatNext.ts monolith (Axis 3/4) #47).
• Key verified finding: several src/<module>/<module>.ts files (e.g. src/imgproc/imgproc.ts) are type-only stubs that throw "Method not implemented"; the real code lives inline in src/jsfeatNext.ts. This is the lead refactoring target.

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