feat: add shell interface, registry, and bash shell emulator

Implement Phase 1.4: replaces the hardcoded banner/timeout stub with a
proper shell system. Adds a Shell interface with weighted registry for
shell selection, a RecordingChannel wrapper (pass-through for now, prep
for Phase 2.3 replay), and a bash-like shell with fake filesystem,
terminal line reader, and command handling (pwd, ls, cd, cat, whoami,
hostname, id, uname, exit). Sessions now log command/output pairs to
the store and record the shell name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/shell/bash/filesystem.go

@@ -0,0 +1,166 @@
+package bash
+
+import (
+	"fmt"
+	"path"
+	"strings"
+)
+
+type fsNode struct {
+	name     string
+	isDir    bool
+	content  string
+	children map[string]*fsNode
+}
+
+type filesystem struct {
+	root *fsNode
+}
+
+func newFilesystem(hostname string) *filesystem {
+	fs := &filesystem{
+		root: &fsNode{name: "/", isDir: true, children: make(map[string]*fsNode)},
+	}
+
+	fs.mkdirAll("/etc")
+	fs.mkdirAll("/root")
+	fs.mkdirAll("/home")
+	fs.mkdirAll("/var/log")
+	fs.mkdirAll("/tmp")
+	fs.mkdirAll("/usr/bin")
+	fs.mkdirAll("/usr/local")
+
+	fs.writeFile("/etc/passwd", "root:x:0:0:root:/root:/bin/bash\n"+
+		"daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin\n"+
+		"www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin\n"+
+		"mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/false\n")
+
+	fs.writeFile("/etc/hostname", hostname+"\n")
+
+	fs.writeFile("/etc/hosts", "127.0.0.1\tlocalhost\n"+
+		"127.0.1.1\t"+hostname+"\n"+
+		"::1\t\tlocalhost ip6-localhost ip6-loopback\n")
+
+	fs.writeFile("/root/.bash_history",
+		"apt update\n"+
+			"apt upgrade -y\n"+
+			"systemctl restart nginx\n"+
+			"tail -f /var/log/syslog\n"+
+			"df -h\n"+
+			"free -m\n"+
+			"netstat -tlnp\n"+
+			"cat /etc/passwd\n")
+
+	fs.writeFile("/root/.bashrc",
+		"# ~/.bashrc: executed by bash(1) for non-login shells.\n"+
+			"export PS1='\\u@\\h:\\w\\$ '\n"+
+			"alias ll='ls -alF'\n"+
+			"alias la='ls -A'\n")
+
+	fs.writeFile("/root/README.txt", "Production server - DO NOT MODIFY\n")
+
+	fs.writeFile("/var/log/syslog",
+		"Jan 12 03:14:22 "+hostname+" systemd[1]: Started Daily apt download activities.\n"+
+			"Jan 12 03:14:23 "+hostname+" systemd[1]: Started Daily Cleanup of Temporary Directories.\n"+
+			"Jan 12 04:00:01 "+hostname+" CRON[12345]: (root) CMD (/usr/local/bin/backup.sh)\n"+
+			"Jan 12 04:00:03 "+hostname+" kernel: [UFW BLOCK] IN=eth0 OUT= SRC=203.0.113.42 DST=10.0.0.5 PROTO=TCP DPT=22\n")
+
+	fs.writeFile("/tmp/notes.txt", "TODO: Update SSL certificates\n")
+
+	return fs
+}
+
+// resolvePath converts a potentially relative path to an absolute one.
+func resolvePath(cwd, p string) string {
+	if !strings.HasPrefix(p, "/") {
+		p = cwd + "/" + p
+	}
+	return path.Clean(p)
+}
+
+func (fs *filesystem) lookup(p string) *fsNode {
+	p = path.Clean(p)
+	if p == "/" {
+		return fs.root
+	}
+
+	parts := strings.Split(strings.TrimPrefix(p, "/"), "/")
+	node := fs.root
+	for _, part := range parts {
+		if node.children == nil {
+			return nil
+		}
+		child, ok := node.children[part]
+		if !ok {
+			return nil
+		}
+		node = child
+	}
+	return node
+}
+
+func (fs *filesystem) exists(p string) bool {
+	return fs.lookup(p) != nil
+}
+
+func (fs *filesystem) isDirectory(p string) bool {
+	n := fs.lookup(p)
+	return n != nil && n.isDir
+}
+
+func (fs *filesystem) list(p string) ([]string, error) {
+	n := fs.lookup(p)
+	if n == nil {
+		return nil, fmt.Errorf("ls: cannot access '%s': No such file or directory", p)
+	}
+	if !n.isDir {
+		return nil, fmt.Errorf("ls: cannot access '%s': Not a directory", p)
+	}
+
+	names := make([]string, 0, len(n.children))
+	for name, child := range n.children {
+		if child.isDir {
+			name += "/"
+		}
+		names = append(names, name)
+	}
+	return names, nil
+}
+
+func (fs *filesystem) read(p string) (string, error) {
+	n := fs.lookup(p)
+	if n == nil {
+		return "", fmt.Errorf("cat: %s: No such file or directory", p)
+	}
+	if n.isDir {
+		return "", fmt.Errorf("cat: %s: Is a directory", p)
+	}
+	return n.content, nil
+}
+
+func (fs *filesystem) mkdirAll(p string) {
+	p = path.Clean(p)
+	parts := strings.Split(strings.TrimPrefix(p, "/"), "/")
+	node := fs.root
+	for _, part := range parts {
+		if node.children == nil {
+			node.children = make(map[string]*fsNode)
+		}
+		child, ok := node.children[part]
+		if !ok {
+			child = &fsNode{name: part, isDir: true, children: make(map[string]*fsNode)}
+			node.children[part] = child
+		}
+		node = child
+	}
+}
+
+func (fs *filesystem) writeFile(p string, content string) {
+	p = path.Clean(p)
+	dir := path.Dir(p)
+	base := path.Base(p)
+
+	fs.mkdirAll(dir)
+	parent := fs.lookup(dir)
+	parent.children[base] = &fsNode{name: base, content: content}
+}