fix(herdr): occupancy reads harness session state, not herdr pane id (B1)

CLIAdapter.Occupancy called a.Usage(s.PaneID), but ClaudeUsage/CodexUsage/
OpenCodeUsage all take a filesystem path to session state, not a herdr pane
id. Every call failed with "open <pane-id>: no such file", and
Coordinator.rotate silently `continue`d past every failure, so occupancy
always looked unmeasurable and rotation never fired.

Add herdr.Session.SessionFile and CLIAdapter.resolveSessionFile:
  - claude: ClaudeSessionFile resolves the transcript by newest-mtime under
    Claude Code's own encoded project directory
    (~/.claude/projects/<abs-worktree-with-/-as-minus>/*.jsonl). This is the
    Phase-1 fallback; the Stop hook's transcript_path (Phase 2) is the
    authoritative source once wired.
  - codex: routes through the existing CodexActiveUsage sqlite/rollout
    discovery instead of the pane id.
  - opencode: resolution needs a live session id from the SSE/status API,
    not derivable from the worktree alone — refuses loudly with a pointer
    to AUDIT.md Phase 1 rather than guessing a path, per the spec's "verify
    against a live session before wiring any trigger" (§5.2.1).

A missing/unreadable session file is now a hard error, not a silent
zero-usage Usage{}. SessionHealth gained Occupancy/OccupancyError fields,
populated every refreshSessionHealth tick, so GET /v1/tasks/{id}/health
makes the number rotation decides on observable before trusting it.

Tests: TestClaudeUsageIsLastTurnNotCumulative guards the exact trap named
in §5.2.1 (large early-turn total, small last-turn usage -> low occupancy).
TestClaudeSessionFileNewestByMtime and TestClaudeSessionFileMissingIsHardError
cover the resolver.

AUDIT.md B1. Live verification against a real Claude Code session (the
spec's own acceptance bar for this phase) still needs to happen on a host
with an actual session — not possible from this sandbox.

internal/herdr/occupancy_test.go

@@ -1,8 +1,11 @@
 package herdr
 
 import (
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestClaudeStopHookUsage(t *testing.T) {
@@ -19,3 +22,74 @@ func TestCodexRolloutFallback(t *testing.T) {
 		t.Fatalf("paths=%v err=%v", paths, err)
 	}
 }
+
+// TestClaudeUsageIsLastTurnNotCumulative guards the exact trap the spec
+// (§5.2.1) calls out by name: occupancy must reflect the current context
+// window (the last turn's usage snapshot), not a running total across many
+// turns. A transcript with one huge early turn and a small final turn must
+// report low occupancy, because Claude Code's own usage lines are already
+// cumulative-per-turn snapshots, not deltas to be summed.
+func TestClaudeUsageIsLastTurnNotCumulative(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "transcript.jsonl")
+	lines := []string{
+		`{"message":{"usage":{"input_tokens":180000,"cache_read_input_tokens":0,"cache_creation_input_tokens":0,"output_tokens":500}}}`,
+		`{"message":{"usage":{"input_tokens":2000,"cache_read_input_tokens":500,"cache_creation_input_tokens":0,"output_tokens":100}}}`,
+	}
+	if err := os.WriteFile(path, []byte(strings.Join(lines, "\n")+"\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	u, err := ClaudeUsage(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	frac := Fraction(u, 200000)
+	if frac > 0.05 {
+		t.Fatalf("occupancy=%v, want low (last-turn usage, not the 180000-token first turn)", frac)
+	}
+}
+
+func TestClaudeSessionFileNewestByMtime(t *testing.T) {
+	worktree := t.TempDir()
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+	abs, err := filepath.Abs(worktree)
+	if err != nil {
+		t.Fatal(err)
+	}
+	enc := strings.ReplaceAll(abs, "/", "-")
+	dir := filepath.Join(home, ".claude", "projects", enc)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	old := filepath.Join(dir, "old.jsonl")
+	newer := filepath.Join(dir, "new.jsonl")
+	if err := os.WriteFile(old, []byte("{}\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(newer, []byte("{}\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	now := time.Now()
+	if err := os.Chtimes(old, now.Add(-time.Hour), now.Add(-time.Hour)); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chtimes(newer, now, now); err != nil {
+		t.Fatal(err)
+	}
+	got, err := ClaudeSessionFile(worktree)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != newer {
+		t.Fatalf("got %s, want newest transcript %s", got, newer)
+	}
+}
+
+func TestClaudeSessionFileMissingIsHardError(t *testing.T) {
+	worktree := t.TempDir()
+	t.Setenv("HOME", t.TempDir())
+	if _, err := ClaudeSessionFile(worktree); err == nil {
+		t.Fatal("expected error for missing session transcript directory, got nil")
+	}
+}