Prepare Agents API 0.1.0 release

[chubes4]

Summary

• Match tool-call and tool-result transcript envelopes by tool_call_id metadata before falling back to legacy FIFO-by-tool-name pairing.
• Keep mediated tool-result tool_call_id in metadata only, avoiding duplicated payload state.
• Make Composer consumption load the same plugin bootstrap via autoload.files and document plugin vs Composer installation for 0.1.x pins.

Verification

• composer validate --strict
• composer test
• homeboy lint --path /Users/chubes/Developer/agents-api@release-0.1.0-prep --extension wordpress
• git diff --check

Release notes

• Closes Upgrade WP_Agent_Tool_Pair_Validator to strict ID-based pairing #189.
• Closes Tool envelope: stop double-injecting tool_call_id into toolResult payload #190.
• Refs Decide vendor mechanism: how downstream consumers ingest canonical #93. The package/docs side is ready here; the first 0.1.0 tag still happens as the release step after merge.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implementing the release-prep code and docs changes, expanding smoke coverage, and running local verification. Chris remains responsible for review and release execution.