feat: Rename 'wisp' to 'ephemeral' in beads API (bd-o18s)

BREAKING CHANGE: API field and CLI command renamed

- types.Issue.Wisp → types.Issue.Ephemeral
- JSON field: "wisp" → "ephemeral"
- CLI: bd wisp → bd ephemeral
- Flags: --wisp → --ephemeral
- ID prefix: wisp → eph

The SQLite column already uses 'ephemeral' so no schema migration needed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/DELETIONS.md

@@ -202,7 +202,7 @@ The 1-hour grace period ensures tombstones propagate even with minor clock drift
 
 ## Wisps: Intentional Tombstone Bypass
 
-**Wisps** (ephemeral issues created by `bd wisp create`) are intentionally excluded from tombstone tracking.
+**Wisps** (ephemeral issues created by `bd ephemeral create`) are intentionally excluded from tombstone tracking.
 
 ### Why Wisps Don't Need Tombstones