Add generic tool runtime metadata

[chubes4]

Summary

• Define optional generic runtime metadata on runtime tool declarations and normalized tool results.
• Propagate declaration runtime metadata through mediated tool execution into tool_execution_results.
• Document canonical duplicate_policy: repeatable and completion_signal: progress semantics with smoke coverage.

Fixes #158.

Tests

• php tests/tool-runtime-smoke.php
• php tests/conversation-loop-tool-execution-smoke.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the focused implementation, tests, documentation, and PR description; Chris remains responsible for review and merge.