Add deterministic agent tool contracts

[Aaronontheweb]

Summary

• Add the validated OpenSpec plan for safer agent tool use.
• Add a PII-free tool-friction fixture with strict mutation and privacy tests.
• Record the actual parent and subagent schema footprint without schema bodies.

Scope

This PR defines deterministic contracts only. Product implementation follows in the stacked PRs.

Deterministic baseline evidence

• A fixed 200-tool synthetic catalog produced 3 parent tool definitions totaling 1,047 serialized UTF-8 bytes.
• The same catalog produced 202 child tool definitions totaling 133,816 serialized UTF-8 bytes.
• The evidence stores aggregate counts and byte sizes only; it contains no schema bodies, raw prompts, logs, paths, session identifiers, or production-derived tool names.
• No hosted behavioral run was used as evidence for this PR.

Validation

• Tool-friction contract tests: 35 passed.
• Model-visible schema-footprint test: passed.
• Strict OpenSpec validation: passed.
• Header verification: passed.
• Slopwatch: zero issues.
• Diff check: passed.