feat(mcp): add fix-agent handoff contract #347

Job	Run time
mutation	1m 42s
Coverage (llvm-cov)	1m 23s
cargo-deny (advisories + licenses)	25s
security	21s
version	6s
lint	1m 21s
test (ubuntu-latest)	45s
test (macos-latest)	45s
test (windows-latest)	56s
	7m 44s

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