fix(mcp): make readiness probe non-invasive

[casey-brooks]

Closes #150

Summary

• Replaced the MCP readiness JSON-RPC initialize probe with a non-invasive TCP listener check against 127.0.0.1:<port>.
• Kept MCP readiness inside agynd and preserved gating before message handling / SDK work starts.
• Updated regression coverage to prove readiness sends no initialize payload and still waits before processing messages.

Validation

• CGO_ENABLED=0 go test ./internal/daemon -run 'TestWaitForMCPServers|TestSyncMessagesWaitsForMCPReadyBeforeHandling' -count=1 — passed: internal/daemon tests passed.
• CGO_ENABLED=0 go test ./... — passed: all Go package tests passed.
• CGO_ENABLED=0 go build ./... — passed: all packages built.
• helm dependency build charts/agynd && helm lint charts/agynd — passed: 1 chart linted, 0 failed.
• AGN_REPO_PATH=/workspace/agn-cli CGO_ENABLED=0 go test -v -count=1 -tags e2e ./test/e2e/ — passed: 3 passed, 0 failed, 0 skipped.

Linting passed with no errors (git diff --check).

[casey-brooks]

Validation Summary

• CGO_ENABLED=0 go test ./internal/daemon -run 'TestWaitForMCPServers|TestSyncMessagesWaitsForMCPReadyBeforeHandling' -count=1 — passed: 6 targeted tests passed, 0 failed, 0 skipped.
• CGO_ENABLED=0 go test ./... — passed: 7 package test suites passed, 0 failed, 0 skipped; cmd/agynd had no test files.
• CGO_ENABLED=0 go build ./... — passed: all packages built.
• helm dependency build charts/agynd && helm lint charts/agynd — passed: 1 chart linted, 0 failed.
• AGN_REPO_PATH=/workspace/agn-cli CGO_ENABLED=0 go test -v -count=1 -tags e2e ./test/e2e/ — passed: 3 tests passed, 0 failed, 0 skipped.
• git diff --check — passed: lint/whitespace check clean.

No deviations from issue #150 requirements.