chore: update AGENTS.md with post-EPIC #88 changes

[randomm]

What

AGENTS.md has 4 stale references after the EPIC #88 open-source readiness work:

1. Architecture table (line ~116): src/pattern.rs → now src/pattern/ module directory
   (mod.rs, builtins.rs, toml.rs)
2. Architecture table (line ~119): src/learn.rs says "Anthropic + OpenAI" → should be
   "Anthropic only" (OpenAI/Cerebras removed in PR fix(#76,#77): add retry loop to oo learn, remove OpenAI/Cerebras #78, v0.3.2)
3. Testing section (line ~71): "~53 tests currently" → should be "~197 tests currently"
4. Architecture notes references to src/pattern.rs (line ~67) → update to module directory

Why

AGENTS.md is the primary orientation document for AI agents working on this codebase.
Stale references cause agents to look for files that do not exist and make wrong
assumptions about the codebase.

Acceptance Criteria

• All references to src/pattern.rs updated to src/pattern/ directory structure
• Learn module description says "Anthropic only" (not OpenAI)
• Test count updated to ~197
• No other stale references remain