Auto-load nearest AGENTS.md into Mini-A runtime instructions

[Copilot]

This PR adds first-class AGENTS.md ingestion so Mini-A automatically applies repository-level agent instructions without requiring rules= wiring. It aligns runtime behavior with Codex/Claude Code/OpenCode-style agent file discovery.

• Instruction discovery and loading

  • Added upward directory traversal to resolve the nearest AGENTS.md from execution context.
  • Added runtime injection of discovered AGENTS.md content into args.rules during init().

• Rule composition semantics

  • Preserves existing user-supplied rules and appends AGENTS-derived guidance as an additional rule block.
  • Stores source provenance (__autoagentspath) for traceability.
  • Guards against duplicate application via a one-time application flag (__autoagentsrulesapplied).

• Core coverage updates

  • Added a core test validating nearest-parent resolution, merge behavior, and idempotency.
  • Registered the new test in tests/coreFunctionality.yaml (jobs + todo).

// new runtime behavior during init()
this._applyAutoAgentsRules(args)

// injected rule shape
"Follow AGENTS.md instructions from <resolved-path>:\n<file-content>"