feat(sqlite): add thread_id columns to contacts schema

Update SQLite schema to include thread_id and thread_type columns.

internal/store/sqlitestore/schema.go

@@ -14,7 +14,7 @@ var schemaSQL string
 
 // SchemaVersion is the current SQLite schema version.
 // Bump this when adding new migration steps below.
-const SchemaVersion = 4
+const SchemaVersion = 5
 
 // migrations maps version → SQL to apply when upgrading FROM that version.
 // schema.sql always represents the LATEST full schema (for fresh DBs).
@@ -42,6 +42,12 @@ UPDATE cron_jobs SET
   deliver_to = COALESCE(json_extract(payload, '$.to'), ''),
   wake_heartbeat = COALESCE(json_extract(payload, '$.wake_heartbeat'), 0)
 WHERE payload IS NOT NULL;`,
+	// Version 4 → 5: add thread_id, thread_type columns to channel_contacts for forum topic support.
+	4: `ALTER TABLE channel_contacts ADD COLUMN thread_id VARCHAR(100);
+ALTER TABLE channel_contacts ADD COLUMN thread_type VARCHAR(20);
+DROP INDEX IF EXISTS idx_channel_contacts_tenant_type_sender;
+CREATE UNIQUE INDEX idx_channel_contacts_tenant_type_sender
+  ON channel_contacts(tenant_id, channel_type, sender_id, COALESCE(thread_id, ''));`,
 	// Version 3 → 4: add subagent_tasks table for subagent lifecycle persistence.
 	3: `CREATE TABLE IF NOT EXISTS subagent_tasks (
     id                TEXT PRIMARY KEY,

internal/store/sqlitestore/schema.sql

@@ -1042,6 +1042,8 @@ CREATE TABLE IF NOT EXISTS channel_contacts (
     avatar_url       TEXT,
     peer_kind        VARCHAR(20),
     contact_type     VARCHAR(20) NOT NULL DEFAULT 'user',
+    thread_id        VARCHAR(100),
+    thread_type      VARCHAR(20),
     metadata         TEXT DEFAULT '{}',
     merged_id        TEXT,
     tenant_id        TEXT NOT NULL REFERENCES tenants(id),
@@ -1049,8 +1051,8 @@ CREATE TABLE IF NOT EXISTS channel_contacts (
     last_seen_at     TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
 );
 
--- tenant-scoped unique (migration 27 Phase I)
-CREATE UNIQUE INDEX IF NOT EXISTS idx_channel_contacts_tenant_type_sender ON channel_contacts(tenant_id, channel_type, sender_id);
+-- tenant-scoped unique including thread_id for topic contacts (migration 35)
+CREATE UNIQUE INDEX IF NOT EXISTS idx_channel_contacts_tenant_type_sender ON channel_contacts(tenant_id, channel_type, sender_id, COALESCE(thread_id, ''));
 CREATE INDEX IF NOT EXISTS idx_channel_contacts_instance ON channel_contacts(channel_instance) WHERE channel_instance IS NOT NULL;
 CREATE INDEX IF NOT EXISTS idx_channel_contacts_merged ON channel_contacts(merged_id) WHERE merged_id IS NOT NULL;
 CREATE INDEX IF NOT EXISTS idx_channel_contacts_search ON channel_contacts(display_name, username);