feat(pydantic-acp): add extension routing and authentication seams

[fswair]

Summary

• add public typed ExtensionRouter and AuthenticationProvider strategies through AdapterConfig
• preserve default behavior and structured ACP errors while negotiating ACP 0.11 terminal authentication capabilities
• verify local stdio and acpremote forwarding, document bridge/router/native ownership, and update skills/API references
• bump the synchronized workspace to 1.6.0 and extend Pydantic AI support through 2.23.0

Validation

• make tests: 702 passed
• Pydantic AI 2.23.0 runtime matrix: 461 passed
• Pydantic AI 2.23.0 ty compatibility: passed
• Pydantic AI 2.9.0-2.22.0 runtime/type matrix: passed
• make check: Ruff, ty, and basedpyright passed
• make check-coverage: 607 passed, 100% line and branch coverage
• make docs-build: strict build passed
• make release-check: 1.6.0 metadata valid

Closes #28

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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