Add long-term memory system with hybrid search

[initializ-mk]

Summary

• Embedder interface + providers: Embedder abstraction with OpenAI (text-embedding-3-small), Gemini, and Ollama (nomic-embed-text) implementations, plus factory with auto-detection and Anthropic fallback handling
• Long-term memory package (forge-core/memory/): file-based storage (daily logs + curated MEMORY.md), text chunker with paragraph/sentence-aware overlap, FileVectorStore with pluggable VectorStore interface, and hybrid search engine combining vector cosine similarity, keyword overlap, and temporal decay (7-day half-life, MEMORY.md evergreen)
• Memory tools: memory_search and memory_get builtin tools enabling agents to query their own long-term memory
• Compactor integration: MemoryFlusher hook captures tool results and decisions before compaction discards old messages, writing them to the daily observation log
• Runner wiring: embedder auto-resolution from LLM provider config, memory.Manager lifecycle, conditional tool registration, and background indexing at startup
• Graceful degradation: no embedder → keyword-only search, no memory dir → skip without crash, corrupted index → rebuild from source files

Enable via memory.long_term: true in forge.yaml or FORGE_MEMORY_LONG_TERM=true env var.

Test plan

• Embedder factory tests (provider routing, defaults)
• FileStore tests (read/write, path traversal prevention, daily logs, MEMORY.md template)
• Chunker tests (paragraph splitting, overlap, empty/large text, unique IDs)
• VectorStore tests (index, search, delete, persistence round-trip, cosine similarity)
• Hybrid search tests (vector+keyword, keyword-only, temporal decay, tokenization)
• Manager tests (full pipeline, mock embedder, graceful degradation on empty index)
• Memory tools tests (memory_search, memory_get with valid/invalid inputs)
• Compactor MemoryFlusher tests (observation capture, SetMemoryFlusher wiring)
• All existing tests pass across forge-core, forge-cli, forge-plugins
• Zero golangci-lint issues across all modules