Support tool source registry composition

[chubes4]

Summary

• add stable source priorities to WP_Agent_Tool_Source_Registry while preserving insertion order within equal priorities
• pass registered sources into the source-order filter and add a per-source gathered-tools filter
• add a focused smoke test covering multiple sources, runtime modes context, source ordering, filter injection, normalization, and duplicate-name precedence
• document the public registry composition contract

Fixes #232

Verification

• php tests/tool-source-registry-smoke.php
• php tests/tool-runtime-smoke.php
• php tests/no-product-imports-smoke.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the registry contract update, smoke tests, docs, and verification; Chris remains responsible for review and merge.