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/proposal.md

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+## Why
+
+A single unquoted word passed to `kb` (e.g., `kb infow`) is silently treated as a note and ingested. This is almost always a mistyped command, not an intentional note. Users lose trust when typos pollute their knowledge base.
+
+## What Changes
+
+- The implicit note shorthand will require **more than one argument** to be treated as a note. A single bare word will be rejected with a helpful error suggesting the user check their command or quote a multi-word note.
+- This is a **BREAKING** change to the implicit note shorthand: `kb singleword` no longer creates a note. Users must write `kb "singleword is important"` or use multiple words.
+
+## Capabilities
+
+### New Capabilities
+
+_(none)_
+
+### Modified Capabilities
+
+- `go-client`: The "Implicit note shorthand" requirement changes to reject single-word bare arguments and print an error instead of submitting them as notes.
+
+## Impact
+
+- **Code**: `client/cmd/root.go` — `RunE` handler for the root command
+- **Tests**: `client/cmd/root_test.go` or equivalent — add/update tests for single-word rejection
+- **Users**: Anyone who intentionally used `kb singleword` as a note shorthand will need to use multiple words or quotes