feat(vault): chat_id isolation for isolated teams

- Add vault_documents.chat_id + composite index (migration 000056)
- Filter vault_search by chat_id when team.workspace_scope=isolated
- Stamp chat_id on AfterWrite/AfterWriteMedia for isolated teams
- Deny cross-chat vault_read in isolated teams (M2 fix)
- RunContext.TeamIsolated flag resolved once per run
- Fallback WorkspaceChatID → ChatID in loop_context for entry points
  that don't set WorkspaceChatID explicitly (WS direct, HTTP, cron)

Fixes cross-chat doc leak where agent in chat A could see vault docs
from chat B within the same isolated team.

internal/store/sqlitestore/schema.sql

@@ -1521,6 +1521,7 @@ CREATE TABLE IF NOT EXISTS vault_documents (
     tenant_id     TEXT NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
     agent_id      TEXT REFERENCES agents(id) ON DELETE SET NULL,
     team_id       TEXT REFERENCES agent_teams(id) ON DELETE SET NULL,
+    chat_id       TEXT,  -- NULL = team-wide (shared / legacy); non-NULL = scoped to specific chat for isolated teams
     scope         TEXT NOT NULL DEFAULT 'personal',
     custom_scope  TEXT,
     path          TEXT NOT NULL,
@@ -1547,6 +1548,7 @@ CREATE INDEX IF NOT EXISTS idx_vault_docs_agent_scope ON vault_documents(agent_i
 CREATE INDEX IF NOT EXISTS idx_vault_docs_type ON vault_documents(agent_id, doc_type);
 CREATE INDEX IF NOT EXISTS idx_vault_docs_hash ON vault_documents(content_hash);
 CREATE INDEX IF NOT EXISTS idx_vault_docs_team ON vault_documents(team_id);
+CREATE INDEX IF NOT EXISTS idx_vault_docs_team_chat ON vault_documents(team_id, chat_id) WHERE team_id IS NOT NULL;
 CREATE INDEX IF NOT EXISTS idx_vault_docs_basename ON vault_documents(tenant_id, path_basename);
 CREATE INDEX IF NOT EXISTS idx_vault_docs_path_prefix ON vault_documents(tenant_id, path);
 CREATE INDEX IF NOT EXISTS idx_vault_docs_delegation