Add Phase 4 knowledge graph: nodes, cells, facts, edges, gRPC service

Build the knowledge graph pillar with the kg package:
- Node: hierarchical notes with parent/children, C2 wiki-style naming,
  shared tag/category pool with artifacts
- Cell: content units (markdown, code, plain) with ordinal ordering
- Fact: EAV tuples with transaction timestamps and retraction support
- Edge: directed graph links (child, parent, related, artifact_link)

Includes schema migration (002_knowledge_graph.sql), protobuf definitions
(kg.proto), full gRPC KnowledgeGraphService implementation, CLI commands
(node create/get), and comprehensive test coverage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

db/migrations/002_knowledge_graph.sql

@@ -0,0 +1,82 @@
+-- Migration 002: Knowledge graph tables
+-- Adds nodes, cells, facts, and graph edges to the unified database.
+
+-- Nodes are entities in the knowledge graph.
+-- A node is either a note or an artifact link.
+CREATE TABLE IF NOT EXISTS nodes
+(
+    id        TEXT PRIMARY KEY,
+    parent_id TEXT,                -- parent node ID (C2 wiki style hierarchy)
+    name      TEXT NOT NULL,       -- human-readable name
+    type      TEXT NOT NULL DEFAULT 'note', -- 'note' or 'artifact_link'
+    created   TEXT NOT NULL,       -- ISO 8601 UTC
+    modified  TEXT NOT NULL,       -- ISO 8601 UTC
+    FOREIGN KEY (parent_id) REFERENCES nodes (id)
+);
+CREATE INDEX IF NOT EXISTS idx_nodes_parent ON nodes (parent_id);
+CREATE INDEX IF NOT EXISTS idx_nodes_name ON nodes (name);
+
+-- Many-to-many junction: nodes <-> tags.
+CREATE TABLE IF NOT EXISTS node_tags
+(
+    node_id TEXT NOT NULL,
+    tag_id  TEXT NOT NULL,
+    FOREIGN KEY (node_id) REFERENCES nodes (id),
+    FOREIGN KEY (tag_id) REFERENCES tags (id)
+);
+
+-- Many-to-many junction: nodes <-> categories.
+CREATE TABLE IF NOT EXISTS node_categories
+(
+    node_id     TEXT NOT NULL,
+    category_id TEXT NOT NULL,
+    FOREIGN KEY (node_id) REFERENCES nodes (id),
+    FOREIGN KEY (category_id) REFERENCES categories (id)
+);
+
+-- Cells are content units within a note.
+-- Inspired by Quiver's cell-based structure.
+CREATE TABLE IF NOT EXISTS cells
+(
+    id       TEXT PRIMARY KEY,
+    node_id  TEXT NOT NULL,
+    type     TEXT NOT NULL DEFAULT 'markdown', -- 'markdown', 'code', etc.
+    contents BLOB,
+    ordinal  INTEGER NOT NULL DEFAULT 0,       -- display order within the node
+    created  TEXT NOT NULL,
+    modified TEXT NOT NULL,
+    FOREIGN KEY (node_id) REFERENCES nodes (id)
+);
+CREATE INDEX IF NOT EXISTS idx_cells_node ON cells (node_id);
+
+-- Facts record EAV relationships with transactional history.
+-- entity + attribute + transaction form the natural key.
+CREATE TABLE IF NOT EXISTS facts
+(
+    id             TEXT PRIMARY KEY,
+    entity_id      TEXT NOT NULL,       -- the subject (node or artifact UUID)
+    entity_name    TEXT NOT NULL,       -- human-readable entity name
+    attribute_id   TEXT NOT NULL,       -- attribute UUID
+    attribute_name TEXT NOT NULL,       -- human-readable attribute name
+    value_contents TEXT NOT NULL,
+    value_type     TEXT NOT NULL,
+    tx_timestamp   INTEGER NOT NULL,    -- Unix epoch of the transaction
+    retraction     INTEGER NOT NULL DEFAULT 0 -- 1 = this fact is retracted
+);
+CREATE INDEX IF NOT EXISTS idx_facts_entity ON facts (entity_id);
+CREATE INDEX IF NOT EXISTS idx_facts_attribute ON facts (attribute_id);
+
+-- Graph edges link nodes to other nodes or artifacts.
+CREATE TABLE IF NOT EXISTS edges
+(
+    id        TEXT PRIMARY KEY,
+    source_id TEXT NOT NULL,       -- source node UUID
+    target_id TEXT NOT NULL,       -- target node or artifact UUID
+    relation  TEXT NOT NULL,       -- 'child', 'parent', 'related', 'artifact_link', etc.
+    created   TEXT NOT NULL,
+    FOREIGN KEY (source_id) REFERENCES nodes (id)
+);
+CREATE INDEX IF NOT EXISTS idx_edges_source ON edges (source_id);
+CREATE INDEX IF NOT EXISTS idx_edges_target ON edges (target_id);
+
+INSERT INTO schema_version (version, applied) VALUES (2, datetime('now'));