feat: añadir sistema de knowledge por agente

Implementa una base de conocimiento persistente por agente siguiendo
el patrón pure core / impure shell:

- pkg/knowledge/: tipos puros (Document, Store interface)
- shell/knowledge/: FileStore con SQLite para indexación y archivos .md
- tools/knowledge.go: 4 tools LLM (search, read, write, list)
- tools/knowledge_test.go: tests unitarios de las tools
- internal/config/schema.go: nuevo KnowledgeToolCfg en ToolsCfg
- agents/runtime.go: inicialización del store y registro de tools
- agents/*/knowledge/about-me.md: documentos semilla para cada agente

Cada agente puede buscar, leer, crear y actualizar documentos de
conocimiento. Los archivos .md viven en agents/<id>/knowledge/ y se
indexan en SQLite (agents/<id>/data/knowledge.db).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/knowledge/store.go

@@ -0,0 +1,25 @@
+package knowledge
+
+import "context"
+
+// Store is the pure interface for knowledge operations.
+// Implemented by shell/knowledge.
+type Store interface {
+	// Search performs full-text search across all documents.
+	Search(ctx context.Context, query string, limit int) ([]SearchResult, error)
+
+	// Get retrieves a document by slug.
+	Get(ctx context.Context, slug string) (*Document, error)
+
+	// Put creates or updates a document (file + index).
+	Put(ctx context.Context, doc Document) error
+
+	// List returns all document slugs with titles.
+	List(ctx context.Context) ([]Document, error)
+
+	// Sync re-indexes all files from disk. Called on startup.
+	Sync(ctx context.Context) error
+
+	// Close releases resources.
+	Close() error
+}

pkg/knowledge/types.go

@@ -0,0 +1,20 @@
+// Package knowledge provides pure types for the agent knowledge base.
+package knowledge
+
+import "time"
+
+// Document represents a knowledge document.
+type Document struct {
+	Slug      string    // filename without extension, e.g. "go-patterns"
+	Title     string    // first H1 line from markdown, or humanized slug
+	Content   string    // full file content
+	UpdatedAt time.Time // file mtime
+}
+
+// SearchResult is a document matched by a search query.
+type SearchResult struct {
+	Slug    string
+	Title   string
+	Snippet string  // relevant fragment with match highlights
+	Rank    float64 // FTS5 relevance score
+}