standards: add scm→a2ml migration script for .bot_directives/

Adds tools/scm-to-a2ml-bot-directives/migrate.jl — a Julia script that
converts hyperpolymath bot-directive SCM files to A2ML (TOML-shaped).

Per user directive 2026-04-17: "absolutely eliminate and update
everywhere the scm stuff, no need for fallback". This is the reusable
migration tool; the estate-wide sweep follows in companion commits
(one per affected repo).

Conversion:
  .bot_directives/<bot>.scm           → .machine_readable/bot_directives/<bot>.a2ml
  .machine_readable/bot_directives/<bot>.scm  → .machine_readable/bot_directives/<bot>.a2ml

Handles three input shapes:
  (bot-directive (bot "X") (scope "Y") (allow (...)) (deny (...)) (notes "..."))
  (bot-directives (version "1.0") (notes "..." "..."))  -- README-style
  (<botname>-config (quality-gates (...)) (<block> (...)) ...)  -- nested

Emits TOML with appropriate sections for nested configs (quality-gates,
zig-analysis, etc. become [quality-gates], [zig-analysis] tables).

Safety:
- Writes .a2ml first, verifies it parses structurally, only then
  deletes the .scm.
- Parse-back failure aborts that file and leaves the .scm in place.
- Legacy .bot_directives/ directory removed when empty.
- Idempotent: second run is a no-op.
- UTF-8 safe tokeniser (em-dashes in README.scm files already encountered).

Usage:
  julia migrate.jl <repo>
  julia migrate.jl --all        # walks the estate
  julia migrate.jl --dry-run <repo>

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath