Mirror runtime status to ~/.antidrift_status

A window-manager status bar can now read a single-line status file: a
glyph plus minutes-remaining (● 24m / ⚠ DRIFT 24m / ● 24m ·? / ◔ planning
/ ✓ review), empty when idle. Rewritten on a one-minute tick, only when
the line changes, and removed on shutdown. Degrades to no file if $HOME
is unresolvable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/statusfile/statusfile.go

@@ -0,0 +1,118 @@
+// Package statusfile mirrors the controller's runtime status into a single-line
+// file (~/.antidrift_status) so an external consumer — a window-manager status
+// bar — can display it with a plain file read.
+package statusfile
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"time"
+
+	"antidrift/internal/domain"
+	"antidrift/internal/session"
+)
+
+// statusFileName is the file written under the user's home directory.
+const statusFileName = ".antidrift_status"
+
+// interval is how often the writer re-renders. Minute granularity is enough for
+// a status bar, so a coarse tick keeps the write rate (and disk churn) low.
+const interval = time.Minute
+
+// DefaultPath resolves ~/.antidrift_status.
+func DefaultPath() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, statusFileName), nil
+}
+
+// Render produces the single status line for the given state. It is empty for
+// Locked/idle so a bar module can hide itself. Drift status strings ("drifting")
+// match the JSON contract the web UI already consumes.
+func Render(st session.State, now time.Time) string {
+	switch st.RuntimeState {
+	case domain.RuntimeActive:
+		timer := remaining(st, now)
+		switch {
+		case st.Drift != nil && st.Drift.Status == "drifting":
+			return "⚠ DRIFT " + timer
+		case st.Drift != nil && st.Drift.Nudge != "":
+			return "● " + timer + " ·?"
+		default:
+			return "● " + timer
+		}
+	case domain.RuntimePlanning:
+		return "◔ planning"
+	case domain.RuntimeReview:
+		return "✓ review"
+	default:
+		return ""
+	}
+}
+
+// remaining formats the minutes left until the commitment deadline, rounded up
+// so a partial minute still reads as a minute and the count hits 0m only at the
+// end. It returns 0m when no deadline is known.
+func remaining(st session.State, now time.Time) string {
+	if st.Commitment == nil || st.Commitment.DeadlineUnixSecs == 0 {
+		return "0m"
+	}
+	left := time.Unix(st.Commitment.DeadlineUnixSecs, 0).Sub(now)
+	if left < 0 {
+		left = 0
+	}
+	mins := int((left + time.Minute - 1) / time.Minute) // ceil
+	return fmt.Sprintf("%dm", mins)
+}
+
+// Writer periodically renders the controller state and writes it to a file,
+// rewriting only when the line changes.
+type Writer struct {
+	path  string
+	state func() session.State
+	now   func() time.Time
+
+	last  string
+	wrote bool
+}
+
+// NewWriter builds a Writer for path, reading state via the given accessor.
+func NewWriter(path string, state func() session.State) *Writer {
+	return &Writer{path: path, state: state, now: time.Now}
+}
+
+// Run writes the status file immediately, then on every tick when the rendered
+// line has changed. It removes the file on ctx cancellation so a stale status
+// does not linger after shutdown.
+func (w *Writer) Run(ctx context.Context) {
+	t := time.NewTicker(interval)
+	defer t.Stop()
+	w.write()
+	for {
+		select {
+		case <-ctx.Done():
+			_ = os.Remove(w.path)
+			return
+		case <-t.C:
+			w.write()
+		}
+	}
+}
+
+func (w *Writer) write() {
+	line := Render(w.state(), w.now())
+	if w.wrote && line == w.last {
+		return
+	}
+	if err := os.WriteFile(w.path, []byte(line), 0o644); err != nil {
+		log.Printf("statusfile: write %s: %v", w.path, err)
+		return
+	}
+	w.last = line
+	w.wrote = true
+}