feat(workspace): metadata-driven indexing/hygiene, document versioning, and patch support

Foundation for the extensible frontend system. Workspace documents now
support metadata flags (skip_indexing, skip_versioning, hygiene config)
via folder-level .config documents and per-file overrides, replacing
hardcoded hygiene targets and indexing behavior.

Key changes:
- DocumentMetadata type with resolution chain (doc → folder .config → defaults)
- Document versioning: auto-saves previous content on write/append/patch
- Workspace patch: search-and-replace editing via memory_write tool
- Hygiene rewrite: discovers cleanup targets from .config metadata
  instead of hardcoded daily/ and conversations/ directories
- memory_read gains version/list_versions params
- memory_write gains metadata/old_string/new_string/replace_all params
- V14 migration adds memory_document_versions table (both PG + libSQL)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

migrations/V14__document_versions.sql

@@ -0,0 +1,23 @@
+-- Document version history for workspace files.
+-- Every content update saves the previous content as a version,
+-- enabling rollback and audit trails.
+
+CREATE TABLE memory_document_versions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    document_id UUID NOT NULL REFERENCES memory_documents(id) ON DELETE CASCADE,
+    version INTEGER NOT NULL,
+    content TEXT NOT NULL,
+    content_hash TEXT NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    changed_by TEXT,
+    UNIQUE(document_id, version)
+);
+
+CREATE INDEX idx_doc_versions_lookup
+    ON memory_document_versions(document_id, version DESC);
+
+-- GIN index on metadata for JSON path queries (used by hygiene to find
+-- .config documents with hygiene.enabled). The metadata column already
+-- exists (V1) but was never indexed.
+CREATE INDEX idx_memory_documents_metadata
+    ON memory_documents USING GIN (metadata jsonb_path_ops);