docs: polish bounded completion discoverability

[Quazmoz]

Summary

Final documentation/discoverability polish for the bounded-completion stack.

This is intentionally docs/routing-only and does not change control-plane runtime semantics, schemas, configuration, or validator behavior.

Changes:

• expose bounded completion in the README and user guide
• document it in root AGENTS.md and the engineering routing index
• clarify across tool integration docs that bounded completion is an orchestration/evidence overlay after selecting the smallest correct domain owner
• preserve authority boundaries: the overlay cannot widen permissions, approvals, or mutation scope
• surface the manual model-picker/distinct-model evidence constraint consistently
• update repository map and validation descriptions to reflect the existing combined suite

Scope

Exactly five files are modified:

• README.md
• docs/user-guide.md
• AGENTS.md
• ENGINEERING_AGENTS_INDEX.md
• docs/tool-integration-guide.md

No runtime/control-plane files are changed.

Validation

Require the repository canonical workflow to pass:

python3 scripts/validate-agentdefaults.py