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/tasks.md

@@ -0,0 +1,5 @@
+## 1. Implementation
+
+- [x] 1.1 Create `client/cmd/reindex.go` with `kb reindex` command, `--yes`/`-y` flag, confirmation prompt matching `remove.go` pattern
+- [x] 1.2 POST to `/api/v1/reindex`, handle human output (`Reindexed N chunks (model: ...)`) and JSON output
+- [x] 1.3 Verify build compiles and command appears in `kb --help`