Fix CLI run defaults and config validation

[vamgan]

Summary

• stop defaulting --target-mode to session so runtime/config defaults are preserved
• stop writing the fake AgentCore runtime ARN during init and warn on placeholder values in doctor
• reject empty or ambiguous run payloads before dispatch
• route wait-mode log writes through injected CLI output

Tests

• npm run typecheck
• npm test -- --run
• npm run build

Fixes #3
Fixes #4
Fixes #5
Fixes #6