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/design.md

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+## Context
+
+The engine's `POST /api/v1/reindex` re-embeds all chunks synchronously and returns `{"chunks_reindexed": N, "model": "..."}`. The client has an established confirmation pattern in `remove.go` using `--yes`/`-y` flag.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Add `kb reindex` with confirmation prompt matching `kb remove` pattern
+- Display human-readable and JSON output
+
+**Non-Goals:**
+- Progress reporting during reindex (engine returns synchronously)
+- Model selection from the client (model is engine-side config)
+
+## Decisions
+
+### 1. Confirmation prompt before reindex
+
+Reindex drops and rebuilds the vector table — destructive if interrupted. Use the same `[y/N]` prompt pattern as `kb remove`, skippable with `--yes`/`-y`.
+
+### 2. Warn that it may take a while
+
+The prompt should mention that reindex re-embeds all chunks, so the user knows it's not instant.