feat: add gate issue type and CLI commands for async coordination (bd-udsi)

Add async gates - coordination primitives for agents to wait on external events
like CI completion, PR merges, timers, or human approval.

Changes:
- Add 'gate' issue type to types.go with gate-specific fields:
  - AwaitType: condition type (gh:run, gh:pr, timer, human, mail)
  - AwaitID: condition identifier
  - Timeout: max wait duration
  - Waiters: mail addresses to notify when gate clears
- Add SQLite migration 027_gate_columns for new fields
- Update all SQLite storage queries to handle gate fields
- Add bd gate commands: create, show, list, close, wait
- All commands support --json output and --no-daemon mode

Closes: bd-2v0f, bd-lz49, bd-u66e

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

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

internal/storage/sqlite/migrations.go

@@ -43,6 +43,7 @@ var migrationsList = []Migration{
 	{"is_template_column", migrations.MigrateIsTemplateColumn},
 	{"remove_depends_on_fk", migrations.MigrateRemoveDependsOnFK},
 	{"additional_indexes", migrations.MigrateAdditionalIndexes},
+	{"gate_columns", migrations.MigrateGateColumns},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -93,6 +94,7 @@ func getMigrationDescription(name string) string {
 		"is_template_column":           "Adds is_template column for template molecules (beads-1ra)",
 		"remove_depends_on_fk":         "Removes FK constraint on depends_on_id to allow external references (bd-zmmy)",
 		"additional_indexes":           "Adds performance optimization indexes for common query patterns (bd-h0we)",
+		"gate_columns":                 "Adds gate columns (await_type, await_id, timeout_ns, waiters) for async coordination (bd-udsi)",
 	}
 	
 	if desc, ok := descriptions[name]; ok {