fix: use OPENAI_API_KEY env var consistently (not undefined OPENAI_KEY)

[FBISiri]

Problem

base_agent.py and synonym_expand/synonym.py read the OpenAI API key from the environment variable OPENAI_KEY, but that variable is never defined anywhere in the project. The README's setup instructions and the rest of base_agent.py use OPENAI_API_KEY:

• README.md (setup): OPENAI_API_KEY="YOUR_API_KEY"
• base_agent.py:149-150: os.environ["OPENAI_API_KEY"] = os.getenv("OPENAI_API_KEY")

But two call sites use the wrong name:

• src/memary/agent/base_agent.py:153 — OpenAIMultiModal(..., api_key=os.getenv("OPENAI_KEY"))
• src/memary/synonym_expand/synonym.py:11 — OpenAI(openai_api_key=os.getenv("OPENAI_KEY"), ...)

Because OPENAI_KEY is undefined, os.getenv("OPENAI_KEY") returns None, so the vision model (gpt-4-vision-preview) and the synonym-expansion LLM are silently initialized with api_key=None and fail with an authentication error at runtime — even when the user has correctly set OPENAI_API_KEY per the README.

Fix

Align both call sites to OPENAI_API_KEY. Two-line, string-only change, no logic change.

- api_key=os.getenv("OPENAI_KEY"),
+ api_key=os.getenv("OPENAI_API_KEY"),

- llm = OpenAI(openai_api_key=os.getenv("OPENAI_KEY"), temperature=0)
+ llm = OpenAI(openai_api_key=os.getenv("OPENAI_API_KEY"), temperature=0)

Verification

grep -rn "OPENAI_KEY" src/ before this change shows the two offending sites are the only places OPENAI_KEY (without _API_) is referenced in src/; every other reference, and the README, uses OPENAI_API_KEY.