feat(agent): gate completion on observable verification conditions

[dxin66]

Summary

• expose optional verify_conditions through both tool schemas
• capture the latest real result produced by each tool
• block no_tool completion while any declared condition remains unsatisfied
• support file existence, file content, command exit code, and tool output checks

Motivation

GenericAgent currently relies mainly on checklist state and response wording when deciding whether a planned task is complete. This allows an agent to report completion without observable evidence.

This change adds a deterministic completion gate for explicitly declared conditions. It does not attempt to solve semantic plan drift or independent reviewer verification.

Verification

• added 7 focused verification-condition tests
• ran 6 existing GA root contract tests
• 13 tests passed on Python 3.12
• validated both tool schema files as JSON
• compiled the changed Python files successfully

Related to #647