chore: migrate .bot_directives/*.scm to .machine_readable/bot_directives/*.a2ml

Estate-wide SCM→A2ML migration per user directive 2026-04-17:
"absolutely eliminate and update everywhere the scm stuff, no need
for fallback".

Migration script: standards/tools/scm-to-a2ml-bot-directives/migrate.jl.
Three bot parsers (sustainabot, panicbot, cipherbot) updated in
companion commit on gitbot-fleet to read .a2ml only — no fallback.

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

Author: hyperpolymath

.bot_directives/README.scm

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: PMPL-1.0-or-later
+# .bot_directives README — migrated from SCM on 2026-04-17
+# Media-Type: application/vnd.bot-directives+a2ml
+
+schema_version = "1.0"
+directive_type = "bot-directives-readme"
+version = "1.0"
+notes = "Repo-specific bot constraints. Bots must follow these directives in addition to global policies."

.bot_directives/echidnabot.scm

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: PMPL-1.0-or-later
+# Bot directive — migrated from SCM on 2026-04-17
+# Media-Type: application/vnd.bot-directive+a2ml
+
+schema_version = "1.0"
+directive_type = "bot-directive"
+bot = "echidnabot"
+scope = "formal verification and fuzzing"
+allow = ["analysis", "fuzzing", "proof checks"]
+deny = ["write to core modules", "write to bindings"]
+notes = "May open findings; code changes require explicit approval"