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/mattn/go-isatty/README.md

@@ -0,0 +1,50 @@
+# go-isatty
+
+[![Godoc Reference](https://godoc.org/github.com/mattn/go-isatty?status.svg)](http://godoc.org/github.com/mattn/go-isatty)
+[![Codecov](https://codecov.io/gh/mattn/go-isatty/branch/master/graph/badge.svg)](https://codecov.io/gh/mattn/go-isatty)
+[![Coverage Status](https://coveralls.io/repos/github/mattn/go-isatty/badge.svg?branch=master)](https://coveralls.io/github/mattn/go-isatty?branch=master)
+[![Go Report Card](https://goreportcard.com/badge/mattn/go-isatty)](https://goreportcard.com/report/mattn/go-isatty)
+
+isatty for golang
+
+## Usage
+
+```go
+package main
+
+import (
+	"fmt"
+	"github.com/mattn/go-isatty"
+	"os"
+)
+
+func main() {
+	if isatty.IsTerminal(os.Stdout.Fd()) {
+		fmt.Println("Is Terminal")
+	} else if isatty.IsCygwinTerminal(os.Stdout.Fd()) {
+		fmt.Println("Is Cygwin/MSYS2 Terminal")
+	} else {
+		fmt.Println("Is Not Terminal")
+	}
+}
+```
+
+## Installation
+
+```
+$ go get github.com/mattn/go-isatty
+```
+
+## License
+
+MIT
+
+## Author
+
+Yasuhiro Matsumoto (a.k.a mattn)
+
+## Thanks
+
+* k-takata: base idea for IsCygwinTerminal
+
+    https://github.com/k-takata/go-iscygpty