mcpx main red after #1910 (f9268e24): TestDeclaredHookFiresOnMatchingKindsOnly + TestSupervisedPumpDetachReattach fail in nix build #2609

The NixOS switch build (2026-10-30 ~01:47 CDT, primary at 819889b) failed in mcpx checkPhase with two test failures that were green at efeea69 (PR #2587 verification):

1. internal/hooks — TestDeclaredHookFiresOnMatchingKindsOnly: hook stdin payload wrong: (empty payload).
2. internal/mcpclient — TestSupervisedPumpDetachReattach: attach 2: supervisor refused attach: already attached.

The only mcpx-touching commit in between is f9268e2 (mcpx plugin: send the session on exec, render execsvc.Result's real fields, #1910). Both failures sit in areas that commit touched (exec session plumbing / hook payloads). This blocks the deploy switch. Fix forward (do not revert #1910's feature); if the tests are timing flakes, harden them and show repeat green runs.