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.go

@@ -30,6 +30,45 @@ func Fraction(u Usage, w int64) float64 {
 	}
 	return f
 }
+// ClaudeSessionFile resolves the transcript file for a Claude Code session
+// running against worktree, by newest-mtime under Claude Code's encoded
+// project directory (~/.claude/projects/<abs-worktree-path-with-/-replaced-
+// by-minus>/*.jsonl). This is the "resolve at lease time" fallback called
+// out in the spec (§5.2.1); the Stop hook's transcript_path (see
+// ClaudeStopHookUsage) is the authoritative source once wired.
+func ClaudeSessionFile(worktree string) (string, error) {
+	abs, err := filepath.Abs(worktree)
+	if err != nil {
+		return "", err
+	}
+	enc := strings.ReplaceAll(abs, "/", "-")
+	home := os.Getenv("HOME")
+	dir := filepath.Join(home, ".claude", "projects", enc)
+	matches, err := filepath.Glob(filepath.Join(dir, "*.jsonl"))
+	if err != nil {
+		return "", err
+	}
+	if len(matches) == 0 {
+		return "", fmt.Errorf("claude: no session transcripts found in %s", dir)
+	}
+	var newest string
+	var newestMod time.Time
+	for _, m := range matches {
+		fi, err := os.Stat(m)
+		if err != nil {
+			continue
+		}
+		if fi.ModTime().After(newestMod) {
+			newestMod = fi.ModTime()
+			newest = m
+		}
+	}
+	if newest == "" {
+		return "", fmt.Errorf("claude: could not stat any session transcript in %s", dir)
+	}
+	return newest, nil
+}
+
 func ClaudeUsage(p string) (Usage, error) {
 	f, e := os.Open(p)
 	if e != nil {