Demonstrate agent-ready repo guidance

[codeforester]

Summary

• Add Quick Start and demo coverage for basectl repo init --agent-ready --dry-run and basectl repo check . --agent-ready.
• Run the agent-ready repository guidance check in CI.
• Register the agent-ready repo guidance contract and enforce it in validation/tests.

Closes #194

Validation

• bats tests/demo_test.bats
• ./tests/validate.sh
• git diff --check
• /Users/rameshhp/work/base/bin/basectl repo check . --agent-ready