Rebuild engine on dragontoothmg and add enterprise repo scaffold

[cjunius]

What & why

Two things in one change:

1. Replace the notnil/chess prototype with a real engine built on
   dylhunn/dragontoothmg, a
   magic-bitboard legal move generator. notnil/chess is convenient but
   allocation-heavy and far too slow for a search that visits millions of
   nodes/sec. Writing our own bitboard generator is a worthwhile project but
   not the point of this one (search + evaluation), so we build on a
   dedicated library. Rationale recorded in docs/adr/0002.
2. Lay the repo out in the standard Go + enterprise-GitHub structure —
   cmd/ + internal/, plus the full set of community/health files, CI, and
   release tooling.

Closes nothing (no tracking issue).

What changed

Engine

• internal/engine
  • perft.go — thin wrappers over dragontoothmg.Perft + timed series +
    divide. Regression tests against published node counts: start position to
    depth 6 (119,060,324) and Kiwipete to depth 4, gated behind -short.
  • eval.go — Evaluate(*Board) int: material + Michniewski piece-square
    tables + bishop-pair bonus, returned relative to the side to move.
  • search.go — iterative-deepening negamax + alpha-beta, quiescence search
    (captures/promotions), MVV-LVA move ordering, mate-distance-aware scoring,
    hard wall-clock budget (checked every 2048 nodes; returns the last
    completed depth).
• internal/uci — line-based UCI loop: uci, isready, ucinewgame,
  position (startpos/fen + moves), go (depth, movetime,
  wtime/btime), stop, d, quit. Emits info + bestmove, renders
  score mate N.
• cmd/gochess — CLI: uci (default), perft <depth> [fen], bench,
  version. version/commit/date injected via -ldflags.

Reviewer note: dragontoothmg king-capture guard

dragontoothmg will generate a king-capturing move when handed a position
where the side not to move is in check, then panic on the empty king
bitboard at the next GenerateLegalMoves. kingCaptureScore plus guards in
Search/negamax/quiesce make the search robust to such positions. This
matters because FEN and UCI input come from GUIs / tournament managers and are
untrusted (see SECURITY.md scope). These positions never arise from legal
play, so there is no search-quality impact.

Repository scaffold

• Docs: rewritten README.md (badges + roadmap), CONTRIBUTING.md,
  SECURITY.md (supported-versions table, private reporting, response SLA),
  SUPPORT.md, MAINTAINERS.md (+ release process), CODE_OF_CONDUCT.md
  (Contributor Covenant 2.1), CHANGELOG.md (Keep a Changelog),
  docs/architecture.md, docs/adr/0001 + 0002.
• .github/: CODEOWNERS, dependabot.yml (gomod + actions + docker),
  PULL_REQUEST_TEMPLATE.md, issue-form templates, workflows:
  • ci.yml — OS x Go matrix, -race, coverage, go mod tidy check,
    govulncheck
  • lint.yml — golangci-lint + gofumpt
  • codeql.yml, release.yml (GoReleaser + SBOM)
• Tooling: Makefile, .golangci.yml, .goreleaser.yaml, multi-stage
  Dockerfile -> distroless, .dockerignore, .editorconfig,
  .gitattributes, updated .gitignore.

Module / license

• Module path: github.com/cjunius/goChess.
• License stays GPL-3.0, which matches dragontoothmg.
• go.sum hashes were taken from the Go checksum database; go mod tidy is a
  no-op.

Verification

Run locally with Go 1.23:

• go build ./..., go vet ./... — clean
• go test -race ./... — pass, including deep perft
• golangci-lint run (v1.61.0) — clean
• gofmt -l . — clean
• gochess perft 6 matches reference counts; UCI loop plays legal moves

Checklist

• Tests added (perft regression + search behavior + UCI)
• Move-generation coverage via perft tests
• CHANGELOG.md updated under ## [Unreleased]
• Chess-strength match results — N/A (first real engine version; no prior baseline)

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