Add same-turn MCP schema activation proof

[mimeding]

Summary

• activate MCP provider tool schemas immediately after a successful capabilities_load result so the next same-turn request can use the newly loaded tool
• update prompt-budget accounting when same-turn schemas are added dynamically
• add a focused proof for buffered MCP provider schema activation

Validation

• git diff --cached --check
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-mcp-same-turn swift test --skip-update --scratch-path /Users/mmeding/Developer/Claude/Projects/osaurus/Packages/OsaurusCore/.build --package-path Packages/OsaurusCore --filter AgentToolLoopTests/capabilitiesLoadActivatesBufferedMCPProviderSchemaForSameTurnUse
• Review findings incorporated: live tool-token accounting now reflects dynamically activated schemas.

Draft gate

• This touches agent-loop behavior and needs local/frontier eval evidence before it should be marked ready for review.