Chunk enrichment: prepend document title to embeddings

Adds enriched_text column to chunks table that prepends document title
(and section header when present) to chunk text. Embeddings and FTS now
use enriched text for better search relevance. Includes schema migration
with backfill for existing data.

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

openspec/changes/archive/2026-03-29-single-word-note-guard/tasks.md

@@ -0,0 +1,10 @@
+## 1. Core Implementation
+
+- [x] 1.1 Update `RunE` in `client/cmd/root.go` to reject single-word bare arguments with an error message and non-zero exit
+- [x] 1.2 Update usage template in `root.go` to reflect that note shorthand requires multiple words
+
+## 2. Tests
+
+- [x] 2.1 Add test: single bare word prints error to stderr and exits non-zero
+- [x] 2.2 Add test: multiple bare words are submitted as a note (existing behavior preserved)
+- [x] 2.3 Add test: zero arguments shows help (existing behavior preserved)