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/specs/engine-api/spec.md

@@ -194,11 +194,11 @@ The engine SHALL provide endpoints to list all tags and manage tags on documents
 
 ### Requirement: Engine status and reindex
 
-The engine SHALL provide status information and support re-embedding all chunks.
+The engine SHALL provide status information and support re-embedding all chunks. The `version` field in the status response SHALL always be present and SHALL reflect the engine's release version as read from the `VERSION` file. This field is the contract used by clients for compatibility checking.
 
 #### Scenario: Get engine status
 - **WHEN** a client sends `GET /api/v1/status`
-- **THEN** the engine SHALL return JSON with model_name, embedding_dim, GPU device info, database stats (document count by type, total chunks, DB size), and queue stats (queued/processing job count)
+- **THEN** the engine SHALL return JSON with `version` (string, from VERSION file), model_name, embedding_dim, GPU device info, database stats (document count by type, total chunks, DB size), and queue stats (queued/processing job count)
 
 #### Scenario: Trigger reindex
 - **WHEN** a client sends `POST /api/v1/reindex`