docs: document openai-compatible provider and FIXME_M9 corrections

Update GUIDE_DEV_PYTHON.md, TODO.md, and AGENTS.md to reflect the
decisions and work done in the FEAT_M9 and FIXME_M9 sessions.

GUIDE_DEV_PYTHON.md:
- Header: add entry in recent updates
- 3.1.2: AI_PROVIDER now documents openai-compatible with
  Literal type; add AI_ALLOW_INSECURE_HTTP row; move decision
  block after table to fix rendering
- 3.2: AISettings code block updated with openai-compatible and
  allow_insecure_http field; decision note extended
- 3.1.3: .env.example adds OpenRouter (HTTPS) and Ollama (HTTP)
  examples, both commented

TODO.md:
- M9 factory line now mentions openai-compatible with URL validation
- Add FEAT_M9 and FIXME_M9 notes after M9 acceptance criteria

AGENTS.md:
- Section 5: new subsection for openai-compatible provider contract
  documenting validation rules, degraded mode, and security constraints

Co-authored-by: opencode/tech-writer anthropic.claude-sonnet-4-5 <anthropic.claude-sonnet-4-5@agents.invalid>

.secrets.baseline

@@ -140,7 +140,7 @@
         "filename": "GUIDE_DEV_PYTHON.md",
         "hashed_secret": "90bd1b48e958257948487b90bee080ba5ed00caa",
         "is_verified": true,
-        "line_number": 4916,
+        "line_number": 4935,
         "is_secret": false
       }
     ],
@@ -177,5 +177,5 @@
       }
     ]
   },
-  "generated_at": "2026-09-07T17:24:40Z"
+  "generated_at": "2026-09-07T17:59:08Z"
 }