Split fetch item reject and defer dispositions

[chubes4]

Summary

• Replace the overloaded fetch skip tool with explicit reject_source and defer_item dispositions.
• Keep rejected sources marked processed while deferrals release the active source claim and leave the item retry eligible.
• Add a focused smoke test covering reject/defer semantics, positive tool descriptions, legacy tool removal, and normal processed marking coverage.

Tests

• php tests/fetch-item-dispositions-smoke.php
• php tests/processed-item-claims-smoke.php
• php -l inc/Core/Steps/Fetch/Tools/FetchItemDispositionTool.php && php -l inc/Core/Steps/Fetch/Handlers/FetchHandler.php && php -l tests/fetch-item-dispositions-smoke.php
• vendor/bin/phpcs inc/Core/Steps/Fetch/Tools/FetchItemDispositionTool.php inc/Core/Steps/Fetch/Handlers/FetchHandler.php tests/fetch-item-dispositions-smoke.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Implementing explicit reject/defer source dispositions for fetch-item tool handling and adding regression tests. Chris remains responsible for review and merge.