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-implicit-note-shorthand/tasks.md

@@ -0,0 +1,19 @@
+## 1. Refactor note submission
+
+- [x] 1.1 Extract note submission logic from `runAdd` into a shared `submitNote` helper (multipart POST, duplicate detection, output formatting)
+
+## 2. Root command shorthand
+
+- [x] 2.1 Add `Args: cobra.ArbitraryArgs` and `RunE` to the root command — join args into a note string, call `submitNote`; show help when no args
+- [x] 2.2 Add `--tags` flag on the root command for note tagging
+
+## 3. Rename add → addfile
+
+- [x] 3.1 Rename command from `add` to `addfile` (`Use: "addfile <path>"`)
+- [x] 3.2 Remove `--note`, `--title`, and `--type` flags from the command
+- [x] 3.3 Add extension validation for single file uploads — reject unsupported extensions with a clear error listing supported types
+
+## 4. Documentation and verification
+
+- [x] 4.1 Update README.md usage section: show `kb "text"` shorthand, rename `add` references to `addfile`
+- [x] 4.2 Verify build compiles, `kb --help` and `kb addfile --help` show expected output