Add command documentation and improve daemon UX

- Added human-readable uptime formatting (51m 59s vs 3119.4 seconds)
- Fixed daemon stop race condition with SIGKILL
- Added markdown docs for 19 commands: blocked, comments, compact, daemon,
  delete, dep, epic, export, import, label, list, quickstart, rename-prefix,
  renumber, reopen, repos, restore, stale, sync
- Closed: bd-159, bd-152, bd-168
- Deleted test issues: bd-144, bd-145

Amp-Thread-ID: https://ampcode.com/threads/T-9f7c3fed-62de-4bcd-a059-8c1b77cdb841
Co-authored-by: Amp <amp@ampcode.com>

commands/export.md

@@ -0,0 +1,24 @@
+---
+description: Export issues to JSONL format
+argument-hint: [-o output-file]
+---
+
+Export all issues to JSON Lines format (one JSON object per line).
+
+## Usage
+
+- **To stdout**: `bd export`
+- **To file**: `bd export -o issues.jsonl`
+- **Filter by status**: `bd export --status open`
+
+Issues are sorted by ID for consistent diffs, making git diffs readable.
+
+## Automatic Export
+
+The daemon automatically exports to `.beads/issues.jsonl` after any CRUD operation (5-second debounce). Manual export is rarely needed unless you need a custom output location or filtered export.
+
+Export is used for:
+- Git version control
+- Backup
+- Sharing issues between repositories
+- Data migration