fix(issue-25): a relative --plan-file is taken against the control root, as its usage line says

Review R1/N1 (reviewer-grok-01, planner-reviewer-codex-01), reproduced before
fixing. The usage line and SKILL.md both promise "relative to the control
root", but parse_args did Path(...).expanduser().resolve(), which is relative
to the caller's cwd. The runner never chdirs to workspace_root, so the two are
routinely different - the documented recipe runs the loop from the control root
while the source root is a worktree.

Two ways it went wrong: a plan that exists only under the control root was not
found at all (exit 1), and a same-named file in the caller's cwd was loaded
instead, quietly putting an unrelated plan into the first implementation
prompt. That second one is the very failure #25 is about, reintroduced through
the option meant to replace it.

Resolve against workspace_root the way jobs_dir already does (Issue #21 R2);
pathlib's `/` leaves an absolute or expanded-home path untouched. Every earlier
test passed an absolute tmp_path, which is why none of them caught this: P3c
covers both failure modes with the control root away from the caller's cwd, and
P3d pins that absolute and ~ paths are not rewritten.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_issue25_plan_reuse.py

@@ -203,6 +203,48 @@ def test_p3b_the_refusal_names_the_resolved_path(ctl, tmp_path, capsys):
     assert str(missing.resolve()) in capsys.readouterr().err
 
 
+def test_p3c_a_relative_plan_file_is_taken_against_the_control_root(ctl, tmp_path, monkeypatch):
+    """R1/N1: the usage line promises control-root-relative, and the runner
+    never chdirs to the control root. Resolving against the caller's cwd looked
+    somewhere else - and a same-named file there won silently, which is the
+    very failure #25 is about."""
+    (ctl / "plans").mkdir()
+    (ctl / "plans" / "named.md").write_text(NEW_PLAN, encoding="utf-8")
+    caller = tmp_path / "caller"
+    (caller / "plans").mkdir(parents=True)
+    monkeypatch.chdir(caller)
+
+    r = _runner("--plan-file", "plans/named.md")
+    r.parse_args()
+    assert Path(r.plan_file) == (ctl / "plans" / "named.md").resolve()
+    assert "NAMED-PLAN-MARKER" in r.current_plan
+
+    # the same name in the caller's cwd must not win
+    (caller / "plans" / "named.md").write_text(OLD_PLAN, encoding="utf-8")
+    r = _runner("--plan-file", "plans/named.md")
+    r.parse_args()
+    assert Path(r.plan_file) == (ctl / "plans" / "named.md").resolve()
+    assert "NAMED-PLAN-MARKER" in r.current_plan
+    assert "OLD-PLAN-MARKER" not in r.current_plan
+
+
+def test_p3d_an_absolute_plan_file_is_left_alone(ctl, tmp_path, monkeypatch):
+    """Anchoring the relative case must not rewrite absolute or ~ paths."""
+    named = tmp_path / "abs-plan.md"
+    named.write_text(NEW_PLAN, encoding="utf-8")
+    monkeypatch.chdir(tmp_path)
+    r = _runner("--plan-file", str(named))
+    r.parse_args()
+    assert Path(r.plan_file) == named.resolve()
+
+    monkeypatch.setenv("HOME", str(tmp_path))
+    home_plan = tmp_path / "home-plan.md"
+    home_plan.write_text(NEW_PLAN, encoding="utf-8")
+    r = _runner("--plan-file", "~/home-plan.md")
+    r.parse_args()
+    assert Path(r.plan_file) == home_plan.resolve()
+
+
 def test_p4_plan_file_conflicts_with_plan_and_needs_a_value(ctl, tmp_path, monkeypatch):
     named = tmp_path / "my-plan.md"
     named.write_text(NEW_PLAN, encoding="utf-8")