feat!: mdflow v3 — markdown agents that evolve

Graduates the v3 branch: engine resolution ladder with default pi, hermetic
pi adapter with Codex auth bridge, cursor-agent and agy engines, md eval
with trust ledger, the ./flows roster convention, the installable agent
skill, and the guardrail/adhoc fixes proven by the field guide.

BREAKING CHANGE: engine: replaces tool:, bare .md files run on the resolved
engine or print as documents, description/route are reserved frontmatter
keys. See the v3 commit series for details.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

# Conflicts:
#	CHANGELOG.md
#	package.json

Author: johnlindquist

.beads/.gitignore

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - main
+      - v3
 
 permissions:
   contents: write

.beads/README.md

@@ -43,3 +43,6 @@ bundle.js
 # Generated bundles for AI upload
 mdflow-bundle.md
 mdflow-core-bundle.md
+
+# packx context-bundler cache
+.packx_cache/