fix(issue-21): review R1-R3 - no env as agent arguments on a reused pane, one jobs dir for a relative MAM_JOBS_DIR, stop keeps the audit-log root

Review 9a1adb46 (planner-reviewer-codex-01):

- R1 [P1] lib.sh new-session: a forwarded binding reached the agent as
  arguments. With env flags the idle-pane branch prefixed `env K=V ...` to
  the command that `herdr agent start ... --kind <agent> -- <args>` passes to
  the agent executable. An idle pane is now reused only when no environment
  has to be set; otherwise the pane is created by pane split / workspace
  create, which take --env. The dead `env` prefix code is removed (its
  send-text fallback would have typed the same line).
- R2 [P2] runner: a relative MAM_JOBS_DIR is resolved once against the
  control root, the way the wrapper (cwd=workspace_root) reads it, and that
  absolute dir is used for delegation, wait, report lookup and cleanup.
- R3 [P2] stop: the row's control_plane.delegate_job_logs_dir is restored
  (exported unless the caller set DELEGATE_JOB_LOGS_DIR), so the
  cancellation history lands in the row's audit-log root.

Tests: T3-1 and T3-3 now assert that the binding arrives as --env on the
pane-creation calls and that no env/K=V token follows `--` in agent start
(fresh and reused pane); T3-4c (audit root on stop) and T2-15 (relative
MAM_JOBS_DIR) are new. The design doc's compatibility table and a create
comment no longer describe the removed idle-pane env path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017R3Eg7MEpSWnm1oB9ZPP6G

.agents/skills/multi-agent-mux-create/scripts/create_session.sh

@@ -205,7 +205,8 @@ fi
 # Issue #21 (item 3): the control-plane binding. Persisted on the row always;
 # forwarded to the pane (leading K=V tokens that lib.sh's new-session turns
 # into --env) only when it differs from what --workspace alone implies.
-# lib.sh splits those flags unquoted (and the idle-pane path evals them), so a
+# lib.sh splits those flags unquoted (and, with env flags, never reuses an
+# idle pane - `agent start --` would pass them to the agent as arguments), so a
 # binding with whitespace that would have to be forwarded is refused.
 CP_JSON="$(mam_control_plane_json "$WORKSPACE" 2>/dev/null || true)"
 CP_PREFIX=""