Add P3b-3 agent registry gate

[lantisprime]

Summary

• add user/project agent registry path, read/write, and exact-hash matching helpers
• add canonical path/project-root helpers and project-root SHA-256 isolation
• add shared canRunAgent runtime gate for built-in, ephemeral, user, and project specs
• export registry/gate helpers from agents/index.ts
• add registry/runtime gate negative and disk-backed tests

Tests

• agents/test-fixtures/run-p3b-3-tests.sh
• scripts/test-security-scan.mjs
• git diff --check

Negative coverage

• unregistered user specs blocked
• raw-byte hash mismatch blocked
• project trust inactive blocked
• project registry root mismatch blocked
• project approvals do not apply across roots
• saved ephemeral-as-user specs blocked until registered
• dangerous candidate and dangerous registry entry blocked
• ephemeral without explicit request blocked
• suspicious ephemeral without confirmation blocked
• ephemeral non-read-only tools blocked

Scope notes

• no child argv construction
• no child Pi execution
• no run_subagent
• no chain mode
• no broad TUI/registration flows