Add P3b-1 agent spec scaffold

[lantisprime]

Summary

• add the P3b-1 core AgentSpec model, constants, defaults, and validators
• add built-in scout, planner, and reviewer specs with read-only tools and safety/output contracts
• add a minimal /agents scaffold command for listing/verifying built-ins without child execution
• add negative validation tests and fake-extension scaffold e2e tests

Validation

• agents/test-fixtures/run-p3b-1-tests.sh
• git diff --check
• npx --yes -p typescript tsc --allowImportingTsExtensions --moduleResolution bundler --module esnext --target es2022 --noEmit agents/lib/specs.ts

Scope notes

• no Markdown discovery
• no registry/trust writes
• no child Pi execution
• no run_subagent
• no chain mode