Support project-level default Ask AI agent

[tomvaillant]

What changed

Adds the project-scoped agents.defaultAgent setting and makes the Ask AI composer use it only when this machine has no locally remembered agent choice.

codex, pi, and terminal-cli:pi are supported. The terminal Pi value lets managed integrations route through the Pi terminal launcher.

Why

Installers can now establish a workspace-appropriate initial agent without overriding a person's existing local selection.

Validation

• corepack pnpm --filter @inkeep/open-knowledge-core typecheck
• corepack pnpm --filter @inkeep/open-knowledge-app typecheck
• corepack pnpm --filter @inkeep/open-knowledge-app exec vitest run src/lib/unified-agent-store.test.ts --pool=forks --maxWorkers=1 --no-file-parallelism --reporter=verbose

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