docs: add design for read_note line numbers by default

Changes withLineNumbers default from false to true so AI assistants
can reference specific line numbers when discussing notes.

docs/plans/2026-01-31-read-note-line-numbers-default-design.md

@@ -0,0 +1,87 @@
+# Design: Line Numbers by Default in `read_note`
+
+**Date:** 2026-01-31
+**Version:** 1.2.1
+
+## Summary
+
+Change `read_note` to return line-numbered content by default (e.g., `1→First line`) to help AI assistants reference specific locations when discussing notes. Add `withLineNumbers: false` to get raw content.
+
+## Motivation
+
+AI assistants can give much more precise references like "line 42 has a typo" rather than vague "in the section about X". Line numbers make file discussions unambiguous.
+
+## Changes
+
+### 1. Default Behavior Change
+
+```typescript
+// Before
+const withLineNumbers = options?.withLineNumbers ?? false;
+
+// After
+const withLineNumbers = options?.withLineNumbers ?? true;
+```
+
+### 2. Apply to Parsed Path
+
+Currently the `parseFrontmatter: true` path ignores line numbers. Add line numbering to the `content` field (and `contentWithoutFrontmatter`) when enabled.
+
+### 3. Schema Update
+
+Update the tool description to say "Default: true" and clarify opt-out with `withLineNumbers: false`.
+
+## Files to Modify
+
+### `src/tools/note-tools.ts`
+- Line 48: Change default from `false` to `true`
+- Lines 125-155: Add line numbering logic to the `parseFrontmatter` path for `content` and `contentWithoutFrontmatter` fields
+- Add `totalLines` to parsed response when line numbers enabled
+
+### `src/tools/index.ts`
+- Lines 51-54: Update schema description to reflect new default
+
+### `tests/note-tools.test.ts`
+- Update existing tests that expect raw content to either:
+  - Explicitly pass `withLineNumbers: false`, or
+  - Update assertions to expect numbered content
+
+## Response Format Examples
+
+### Before (current default)
+```json
+{
+  "content": "# Title\nSome content",
+  "wordCount": 3,
+  "versionId": "abc123"
+}
+```
+
+### After (new default)
+```json
+{
+  "content": "1→# Title\n2→Some content",
+  "totalLines": 2,
+  "wordCount": 3,
+  "versionId": "abc123"
+}
+```
+
+### Opt-out (`withLineNumbers: false`)
+```json
+{
+  "content": "# Title\nSome content",
+  "wordCount": 3,
+  "versionId": "abc123"
+}
+```
+
+## Breaking Change
+
+This changes the default response format. MCP clients that parse `content` expecting raw text will need to either:
+- Update their parsing to handle line-numbered format
+- Explicitly pass `withLineNumbers: false`
+
+## Version
+
+Bump to 1.2.1.