docs: reduce SPEC.md to a language-only specification

Abandon the milestone-driven organisation of SPEC.md. The document now
contains only design philosophy (§1) and the language specification
proper (lexical structure, types, ownership/borrow rules, grammar,
semantics, module/unit semantics, and the minimal stdlib surface the
language itself depends on).

Removed:
- All milestone content (M1-M12: descriptions, completion criteria,
  ordering) and the roadmap/schedule framing.
- Compiler implementation directives: bootstrap strategy, pipeline,
  directory layout, C emission rules, own.c algorithm (§11 in full).
- Test/fixture plans and pass/fail/run16/boot fixture listings (§12
  in full).
- Build-driver/tooling detail that isn't part of the language itself:
  import-root search and ambiguity resolution, .fei cache/hash format,
  the full CLI flag reference table, C-backend evaluation-order
  lowering notes, and the generic-instance C emission ordering.
- SPEC.AUDIT.md entirely (git rm) — the accumulated change log for the
  old milestone-driven spec no longer applies.

Kept and reorganised: §2-§9 (targets, lexical structure, type system,
ownership/borrow rules R1-R11, grammar, statement/expression
semantics, unit/import/visibility semantics, generics) are otherwise
unchanged in wording. §10 (stdlib) is now a short placeholder noting
the stdlib spec is pending, while retaining the minimal surface the
language rules and builtins directly reference (core.Error, str alias
methods, mem.replace/create/destroy/alloc_slice, io.Writer/Reader,
fmt.fmt_*, sys.exit/on_exit). §13 (excluded features) is renumbered to
§11 and kept as-is since it documents language-design decisions, not
implementation.

AGENTS.md's document map is updated to drop the SPEC.AUDIT.md row and
reflect that SPEC.md is now language-only.

Implementation and stdlib specs are intended to be written as separate
documents going forward.

AGENTS.md

@@ -7,8 +7,7 @@ DOS용 시스템 프로그래밍 언어 Ferro와 그 컴파일러 `fec`. 규범
 
 | 파일 | 역할 |
 |---|---|
-| `SPEC.md` | 언어 명세 + 구현 지시서. 유일한 규범 문서 |
-| `SPEC.AUDIT.md` | 명세 변경의 문제·결정·근거·구현 영향 누적 로그 |
+| `SPEC.md` | 언어 명세. 유일한 규범 문서. 구현 지시서와 표준 라이브러리 명세는 별도 문서 |
 | `tools/README.md` | 호스트 요구사항, 최초 셋업, 자동화 구조 |
 
 개발 환경과 테스트 명령 목록·플래그는 CLI로 확인한다.