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/ARCHITECTURE.md

@@ -275,7 +275,7 @@ open ──▶ in_progress ──▶ closed
 
 ```
 ┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
-│ bd wisp create  │───▶│  Wisp Issues    │───▶│  bd mol squash  │
+│ bd ephemeral create  │───▶│  Wisp Issues    │───▶│  bd mol squash  │
 │ (from template) │    │  (local-only)   │    │  (→ digest)     │
 └─────────────────┘    └─────────────────┘    └─────────────────┘
 ```