refactor: deduplicate optional extras using self-referencing dependencies

[basnijholt]

Summary

• Introduces a vectordb base extra that captures the 8 shared dependencies between rag and memory (chromadb, pydantic-ai, openai, onnxruntime, huggingface-hub, transformers, watchfiles, and fastapi)
• Replaces repeated fastapi[standard] in 7 extras (rag, memory, faster-whisper, mlx-whisper, whisper-transformers, piper, kokoro) with agent-cli[server] self-references
• Reduces rag and memory from ~8 lines each to 1-line self-references plus their unique dep

Before:

rag = [
    "pydantic-ai-slim[openai,google]>=0.1.1",
    "fastapi[standard]",
    "chromadb>=0.4.22",
    "openai>=1.0.0",
    "onnxruntime>=1.17.0",
    "huggingface-hub>=0.20.0",
    "transformers>=4.30.0",
    "watchfiles>=0.21.0",
    "markitdown[docx,pdf,pptx]>=0.1.3",
]
memory = [
    "pydantic-ai-slim[openai,google]>=0.1.1",
    "fastapi[standard]",
    "chromadb>=0.4.22",
    "openai>=1.0.0",
    "onnxruntime>=1.17.0",
    "huggingface-hub>=0.20.0",
    "transformers>=4.30.0",
    "watchfiles>=0.21.0",
    "pyyaml>=6.0.0",
]

After:

vectordb = [
    "agent-cli[server]",
    "pydantic-ai-slim[openai,google]>=0.1.1",
    "chromadb>=0.4.22",
    "openai>=1.0.0",
    "onnxruntime>=1.17.0",
    "huggingface-hub>=0.20.0",
    "transformers>=4.30.0",
    "watchfiles>=0.21.0",
]
rag = ["agent-cli[vectordb]", "markitdown[docx,pdf,pptx]>=0.1.3"]
memory = ["agent-cli[vectordb]", "pyyaml>=6.0.0"]

Uses the same self-referencing pattern already established by audio = [..., "agent-cli[wyoming]"].

Test plan

• All 990 tests pass
• All pre-commit hooks pass (including check-extras-sync, sync-requirements, uv lock)
• _extras.json and _requirements/*.txt regenerated correctly
• vectordb added to install-extras help text and docstring

[basnijholt]

pre-commit.ci: please retry