feat: SynapseKit v0.6.4

[AmitoVrito]

Summary

• DocxLoader — load Microsoft Word (.docx) files (pip install synapsekit[docx])
• MarkdownLoader — load Markdown files with optional YAML frontmatter stripping (stdlib)
• HyDERetriever — Hypothetical Document Embeddings retrieval strategy
• ShellTool — async shell command execution with timeout and allowed_commands whitelist
• SQLSchemaInspectionTool — inspect database schema (list_tables, describe_table)
• FilesystemLLMCache — persistent file-based LLM cache using JSON files (cache_backend="filesystem")
• JSONFileCheckpointer — file-based graph checkpointing using JSON files
• COST_TABLE — updated with GPT-4.1, o3/o4-mini, Gemini 2.5, DeepSeek, Groq models
• Version bumped to 0.6.4 — 587 tests passing, lint clean

Test plan

• All 587 tests pass (uv run pytest tests/ -q)
• Lint clean (uv run ruff check src/ tests/)
• CI passes