Reindex command, implicit note shorthand, add→addfile rename

- Add `kb reindex` command with confirmation prompt and --yes flag
- Add implicit note shorthand: `kb "my note"` submits a note directly
- Rename `add` to `addfile`, remove --note/--title/--type flags
- Add client-side file extension validation before upload
- Add `kb examples` command for common usage patterns
- Update README, SKILL.md, and main specs
- Archive completed changes and sync delta specs

BREAKING: `kb add` renamed to `kb addfile`, `kb add --note` replaced by `kb "text"`

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

openspec/changes/archive/2026-03-29-client-reindex-command/specs/go-client/spec.md

@@ -0,0 +1,25 @@
+## ADDED Requirements
+
+### Requirement: Reindex command
+
+The client SHALL provide a `kb reindex` command that triggers re-embedding of all chunks on the engine. The command SHALL prompt for confirmation before proceeding.
+
+#### Scenario: Reindex with confirmation
+- **WHEN** the user runs `kb reindex`
+- **THEN** the client SHALL display a warning that all chunks will be re-embedded and prompt `Reindex all chunks? This will re-embed everything. [y/N]`. If confirmed, it SHALL POST to `/api/v1/reindex` and display the result.
+
+#### Scenario: Reindex with skip confirmation
+- **WHEN** the user runs `kb reindex --yes`
+- **THEN** the client SHALL skip the confirmation prompt and POST to `/api/v1/reindex` immediately
+
+#### Scenario: Reindex cancelled
+- **WHEN** the user runs `kb reindex` and responds with anything other than `y` or `yes`
+- **THEN** the client SHALL print `Cancelled.` and exit with code 0
+
+#### Scenario: Reindex human output
+- **WHEN** the reindex completes successfully with default format
+- **THEN** the client SHALL print `Reindexed N chunks (model: <model_name>)`
+
+#### Scenario: Reindex JSON output
+- **WHEN** the user runs `kb reindex --yes --format json`
+- **THEN** the client SHALL output the raw JSON response from the engine