feat(01-01): scaffold Go project with types

- shell.nix with go and gopls for NixOS dev environment
- go.mod for github.com/pieMusic/vmux module
- types.go with Process, Session, SessionState (Working/NeedsInput/Idle/Unknown)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

go.mod

@@ -0,0 +1,3 @@
+module github.com/pieMusic/vmux
+
+go 1.25

shell.nix

@@ -0,0 +1,4 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  buildInputs = with pkgs; [ go gopls ];
+}

types.go

@@ -0,0 +1,42 @@
+package main
+
+// SessionState represents the current activity state of a Claude Code session.
+type SessionState int
+
+const (
+	Working    SessionState = iota
+	NeedsInput
+	Idle
+	Unknown
+)
+
+func (s SessionState) String() string {
+	switch s {
+	case Working:
+		return "Working"
+	case NeedsInput:
+		return "Needs Input"
+	case Idle:
+		return "Idle"
+	default:
+		return "Unknown"
+	}
+}
+
+// Process represents a Claude Code OS process detected via /proc.
+type Process struct {
+	PID int
+	Cmd []string // cmdline arguments
+	Cwd string   // readlink /proc/PID/cwd
+}
+
+// Session represents a Claude Code session enriched with JSONL metadata.
+type Session struct {
+	Process   Process
+	SessionID string
+	GitBranch string
+	State     SessionState
+	Preview   string // last lines of output
+	CwdPath   string // process cwd
+	Worktree  string // git worktree (may differ from cwd)
+}