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/github.com/google/uuid/marshal.go

@@ -0,0 +1,38 @@
+// Copyright 2016 Google Inc.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package uuid
+
+import "fmt"
+
+// MarshalText implements encoding.TextMarshaler.
+func (uuid UUID) MarshalText() ([]byte, error) {
+	var js [36]byte
+	encodeHex(js[:], uuid)
+	return js[:], nil
+}
+
+// UnmarshalText implements encoding.TextUnmarshaler.
+func (uuid *UUID) UnmarshalText(data []byte) error {
+	id, err := ParseBytes(data)
+	if err != nil {
+		return err
+	}
+	*uuid = id
+	return nil
+}
+
+// MarshalBinary implements encoding.BinaryMarshaler.
+func (uuid UUID) MarshalBinary() ([]byte, error) {
+	return uuid[:], nil
+}
+
+// UnmarshalBinary implements encoding.BinaryUnmarshaler.
+func (uuid *UUID) UnmarshalBinary(data []byte) error {
+	if len(data) != 16 {
+		return fmt.Errorf("invalid UUID (got %d bytes)", len(data))
+	}
+	copy(uuid[:], data)
+	return nil
+}