Add web UI and worker capture/approval command channel

Introduces the browser-facing surface and the worker-side protocol that
backs it:

- internal/ui: joined read model plus per-task lifecycle and approval
  controls, kept separate from the raw endpoints workers and harnesses
  depend on.
- internal/webui + web/: Vite/React app, build output embedded via
  go:embed and served as an SPA fallback.
- federation: per-(worker, task) captures with a monotonic revision that
  advances only when pane text actually changes, and a command queue
  restricted to grant_approval / deny_approval, each bound to the capture
  revision the operator acted on.
- orchestra-worker: publishes captures and executes commands only after
  re-reading the pane and confirming the revision still matches. Sends
  keystrokes only for a visible y/n prompt or OpenCode's fully labelled
  selector, and refuses to deny through that selector rather than guess
  at unobservable navigation.

This is the ownership boundary AUDIT.md's B14 and B17 call for: approval
becomes an explicit, revision-bound operation executed by the worker that
owns the pane, instead of a side effect of prompting over a
coordinator-driven remote socket.

Also ignores the web build inputs and outputs. node_modules ships vendored
Go packages, so go build and go test walk into it if it is merely
untracked; both node_modules and .node_modules are excluded.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01535A3Y8RtkAi8wYuWhtkEd

internal/webui/webui.go

@@ -0,0 +1,37 @@
+// Package webui embeds the compiled browser application in the Orchestra binary.
+package webui
+
+import (
+	"embed"
+	"io/fs"
+	"net/http"
+	"path"
+	"strings"
+)
+
+//go:embed assets/* assets/assets/*
+var files embed.FS
+
+// Handler serves immutable fingerprinted assets and falls back to the SPA for
+// browser routes. API routes are deliberately not handled here.
+func Handler() http.Handler {
+	root, _ := fs.Sub(files, "assets")
+	static := http.FileServer(http.FS(root))
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		p := strings.TrimPrefix(path.Clean(r.URL.Path), "/")
+		if p != "" && p != "." {
+			if _, err := fs.Stat(root, p); err == nil {
+				if strings.Contains(p, "/assets/") || strings.HasPrefix(p, "assets/") {
+					w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
+				}
+				static.ServeHTTP(w, r)
+				return
+			}
+		}
+		w.Header().Set("Cache-Control", "no-cache")
+		b, err := fs.ReadFile(root, "index.html")
+		if err != nil { http.Error(w, "web UI unavailable", http.StatusInternalServerError); return }
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		_, _ = w.Write(b)
+	})
+}