Support wisps in nodb mode for lightweight Gas Town rigs

Enable proper wisp support in --no-db mode to allow lightweight Gas Town rigs (including HQ/Mayor) to run without SQLite overhead.

## Motivation
Gas Town HQ and simple rigs could benefit from nodb mode's simplicity. Currently wisps don't work properly in nodb mode.

## Proposed Design

### Phase 1: Session-scoped wisps
- Memory storage already has Wisp field on Issue
- Wisps exist only for the session lifetime
- Acceptable for patrol cycles that complete within a session
- Depends on: bd-9avq (fix the leak)

### Phase 2: Optional wisp persistence
- Add `.beads/wisps.jsonl` for wisp storage (gitignored)
- Load wisps from this file on nodb startup
- Save wisps to this file on nodb exit
- Wisps persist across restarts but don't sync via git

### Phase 3: Verify mol commands
- Test mol squash, burn, bond work with memory storage
- These currently check for SQLite storage type
- May need interface-based approach instead of type assertions

## Files affected
- cmd/bd/nodb.go - wisp filtering and optional persistence
- internal/storage/memory/ - verify wisp field handling
- cmd/bd/mol_*.go - verify memory storage compatibility
- .beads/.gitignore - add wisps.jsonl

Simplified scope: just fix the leak (bd-9avq). Session-scoped wisps are acceptable - they're ephemeral by design. Persistence can be added later if needed.