Context

The "default agent" concept (default: true on AgentConfig, resolveDefaultAgentId()) silently routes messages to an arbitrary agent when routing fails. This hides configuration errors. See docs/scope/gut-default-agent.md for the full scope.

This is the foundational issue — all other gut-default-agent issues depend on this landing first.

What

1. Remove default?: boolean from AgentConfig type (src/config/types.agents.ts:10)
2. Remove default: z.boolean().optional() from zod schema (src/config/zod-schema.agent-runtime.ts:692)
3. Add resolveSoleAgentId(cfg): string | null — returns agent ID if exactly one agent, null otherwise
4. Add requireSoleAgentId(cfg): string — throws if zero or multiple agents
5. Deprecate resolveDefaultAgentId() as a shim delegating to the new function (temporary, removed once all callers migrate)

Acceptance Criteria

• AgentConfig.default field removed from TypeScript type and zod schema
• resolveSoleAgentId() returns agent ID for single-agent config (regardless of name)
• resolveSoleAgentId() returns null for zero or multiple agents
• requireSoleAgentId() throws descriptive error for zero or multiple agents
• Existing tests updated; CI passes (build, test, lint, docs)

Files

• src/config/types.agents.ts
• src/config/zod-schema.agent-runtime.ts
• src/agents/agent-scope.ts
• src/agents/agent-scope.test.ts
• src/config/sessions/main-session.ts — inline default-resolution at line 20: agents.find((agent) => agent?.default)?.id (bypasses resolveDefaultAgentId entirely)
• src/config/legacy.shared.ts — resolveDefaultAgentIdFromRaw at line 94 reads entry.default === true
• src/config/legacy.rules.ts — migration rule at line 112 referencing agents.list[].default

Migration Scope

resolveDefaultAgentId has ~50 production call sites across ~30 files (117 total references including imports, tests, and the definition itself). This issue creates the replacement functions (resolveSoleAgentId, requireSoleAgentId); dependent issues (#1576–#1581) migrate subsets of callers. Remaining callers (~20 files not covered by #1576–#1581) will need a follow-up issue.