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/commands.go

@@ -0,0 +1,119 @@
+package bash
+
+import (
+	"fmt"
+	"runtime"
+	"sort"
+	"strings"
+)
+
+type shellState struct {
+	cwd      string
+	username string
+	hostname string
+	fs       *filesystem
+}
+
+type commandResult struct {
+	output string
+	exit   bool
+}
+
+func dispatch(state *shellState, line string) commandResult {
+	fields := strings.Fields(line)
+	if len(fields) == 0 {
+		return commandResult{}
+	}
+
+	cmd := fields[0]
+	args := fields[1:]
+
+	switch cmd {
+	case "pwd":
+		return commandResult{output: state.cwd}
+	case "whoami":
+		return commandResult{output: state.username}
+	case "hostname":
+		return commandResult{output: state.hostname}
+	case "id":
+		return cmdID(state)
+	case "uname":
+		return cmdUname(state, args)
+	case "ls":
+		return cmdLs(state, args)
+	case "cd":
+		return cmdCd(state, args)
+	case "cat":
+		return cmdCat(state, args)
+	case "exit", "logout":
+		return commandResult{exit: true}
+	default:
+		return commandResult{output: fmt.Sprintf("%s: command not found", cmd)}
+	}
+}
+
+func cmdID(state *shellState) commandResult {
+	return commandResult{
+		output: fmt.Sprintf("uid=0(%s) gid=0(%s) groups=0(%s)", state.username, state.username, state.username),
+	}
+}
+
+func cmdUname(state *shellState, args []string) commandResult {
+	if len(args) > 0 && args[0] == "-a" {
+		return commandResult{
+			output: fmt.Sprintf("Linux %s 5.15.0-89-generic #99-Ubuntu SMP Mon Oct 30 20:42:41 UTC 2023 %s GNU/Linux", state.hostname, runtime.GOARCH),
+		}
+	}
+	return commandResult{output: "Linux"}
+}
+
+func cmdLs(state *shellState, args []string) commandResult {
+	target := state.cwd
+	if len(args) > 0 {
+		target = resolvePath(state.cwd, args[0])
+	}
+
+	names, err := state.fs.list(target)
+	if err != nil {
+		return commandResult{output: err.Error()}
+	}
+
+	sort.Strings(names)
+	return commandResult{output: strings.Join(names, "  ")}
+}
+
+func cmdCd(state *shellState, args []string) commandResult {
+	target := "/root"
+	if len(args) > 0 {
+		target = resolvePath(state.cwd, args[0])
+	}
+
+	if !state.fs.exists(target) {
+		return commandResult{output: fmt.Sprintf("bash: cd: %s: No such file or directory", args[0])}
+	}
+	if !state.fs.isDirectory(target) {
+		return commandResult{output: fmt.Sprintf("bash: cd: %s: Not a directory", args[0])}
+	}
+
+	state.cwd = target
+	return commandResult{}
+}
+
+func cmdCat(state *shellState, args []string) commandResult {
+	if len(args) == 0 {
+		return commandResult{}
+	}
+
+	var parts []string
+	for _, arg := range args {
+		p := resolvePath(state.cwd, arg)
+		content, err := state.fs.read(p)
+		if err != nil {
+			parts = append(parts, err.Error())
+		} else {
+			parts = append(parts, strings.TrimRight(content, "\n"))
+		}
+	}
+
+	return commandResult{output: strings.Join(parts, "\n")}
+}