feat(ecosystem): compliant Praxis/Hexis integration + vendored build

Bring the Nexus/Praxis/Hexis integration in line with
MAVEN_ECOSYSTEM_ARCHITECTURE.md:

- Praxis over HTTP: drop the in-process praxis.db open (praxisstore/
  praxistools) and call praxisd's /api/v1/tools/* API via a new praxisClient.
  Honors the "no component reads another's DB" invariant (AC#12).
  PraxisConfig.DBPath -> URL.
- Hexis confirmation gate: mutating capabilities (ReadOnly=false) now park a
  bound pendingHexis confirmation and require a spoken "да" before executing;
  read-only run immediately (AC#7, no auto attention->action).
- Capability safety: >1 verb match is ambiguous -> ask instead of firing the
  first; ambiguous Nexus resolution asks for clarification (AC#2).
- Correlation IDs on Hexis execute, recorded in the cross-service trace.
- Bug: importance arrives as JSON float64 over HTTP, not int.
- Tests: confirm-gate, decline, read-only, and ambiguity paths.

Build: vendor/ bakes in the hexis client (replace-directed at a sibling repo
outside the Docker context); Dockerfile builds from vendor and no longer
`go mod download`s the unreachable replace paths.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

vendor/golang.org/x/sys/unix/dev_zos.go

@@ -0,0 +1,28 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build zos && s390x
+
+// Functions to access/create device major and minor numbers matching the
+// encoding used by z/OS.
+//
+// The information below is extracted and adapted from <sys/stat.h> macros.
+
+package unix
+
+// Major returns the major component of a z/OS device number.
+func Major(dev uint64) uint32 {
+	return uint32((dev >> 16) & 0x0000FFFF)
+}
+
+// Minor returns the minor component of a z/OS device number.
+func Minor(dev uint64) uint32 {
+	return uint32(dev & 0x0000FFFF)
+}
+
+// Mkdev returns a z/OS device number generated from the given major and minor
+// components.
+func Mkdev(major, minor uint32) uint64 {
+	return (uint64(major) << 16) | uint64(minor)
+}