feat: add type: event for operational state changes (bd-ecmd)

Adds support for event beads that capture operational state transitions
as immutable records. Events are a new issue type with fields:
- event_kind: namespaced category (patrol.muted, agent.started)
- actor: entity URI who caused the event
- target: entity URI or bead ID affected
- payload: event-specific JSON data

This enables:
- bd activity --follow showing events
- bd list --type=event --target=agent:deacon
- Full audit trail for operational state
- HOP-compatible transaction records

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

internal/storage/sqlite/schema.go

@@ -37,6 +37,11 @@ CREATE TABLE IF NOT EXISTS issues (
     is_template INTEGER DEFAULT 0,
     -- Molecule type field (bd-oxgi)
     mol_type TEXT DEFAULT '',
+    -- Event fields (bd-ecmd)
+    event_kind TEXT DEFAULT '',
+    actor TEXT DEFAULT '',
+    target TEXT DEFAULT '',
+    payload TEXT DEFAULT '',
     -- NOTE: replies_to, relates_to, duplicate_of, superseded_by removed per Decision 004
     -- These relationships are now stored in the dependencies table
     -- closed_at constraint: closed issues must have it, tombstones may retain it from before deletion