fix(subdirectory_hints): catch RuntimeError from Path.expanduser()

`pathlib.Path('~user').expanduser()` raises RuntimeError when the
tilde-expansion can't resolve the user (e.g. `~500-700` where the LLM
meant "approximately 500-700" rather than a path). The hint walker's
existing `except (OSError, ValueError):` clauses do not catch
RuntimeError, so it escapes through the tool dispatcher and surfaces
in the conversation loop as a misleading

    Error during OpenAI-compatible API call #N:
    Could not determine home directory.

Reproduced across three unrelated models (openai/gpt-5-mini,
openai/gpt-5.1-codex, deepseek/deepseek-v4-flash) on terminal-tool
commands containing literal tildes in non-path contexts — common in
LLM output ("~500 agencies", "~45,000 CVEs", "~80/hr blended rate").

Reproduction (one-liner):
    >>> from pathlib import Path
    >>> Path("~500-700").expanduser()
    RuntimeError: Could not determine home directory.

Fix: extend the three `except` clauses in
agent/subdirectory_hints.py to also catch RuntimeError:

  line 138 (_add_path_candidate's outer catch around the Path().expanduser() call)
  lines 198+202 (_load_hints_for_directory's nested catches around hint_path.relative_to(Path.home()))

Tests: tests/agent/test_subdirectory_hints_tilde.py adds three cases
covering: tilde-as-approximately in heredoc commands, ~unknown_user paths,
and a regression guard that legitimate ~/path expansion still works.

agent/subdirectory_hints.py

@@ -144,7 +144,7 @@ class SubdirectoryHintTracker:
                 if parent == p:
                     break  # filesystem root
                 p = parent
-        except (OSError, ValueError):
+        except (OSError, ValueError, RuntimeError):
             pass
 
     def _extract_paths_from_command(self, cmd: str, candidates: Set[Path]):
@@ -241,11 +241,11 @@ class SubdirectoryHintTracker:
                 rel_path = str(hint_path)
                 try:
                     rel_path = str(hint_path.relative_to(self.working_dir))
-                except ValueError:
+                except (ValueError, RuntimeError):
                     try:
                         rel_path = str(hint_path.relative_to(Path.home()))
                         rel_path = "~/" + rel_path
-                    except ValueError:
+                    except (ValueError, RuntimeError):
                         pass  # keep absolute
                 found_hints.append((rel_path, content))
                 # First match wins per directory (like startup loading)

tests/agent/test_subdirectory_hints_tilde.py

@@ -0,0 +1,54 @@
+"""Regression tests for the home-directory RuntimeError bug.
+
+Without the fix to ``agent/subdirectory_hints.py`` (add ``RuntimeError`` to
+the three ``except`` clauses around ``Path.expanduser()`` /
+``Path.home()``), the first two tests raise ``RuntimeError`` from inside
+the hint walker on POSIX systems.
+
+These tests use pytest's built-in ``tmp_path`` fixture and intentionally
+do not depend on the richer ``project`` fixture from
+``test_subdirectory_hints.py`` so the file is runnable standalone.
+"""
+
+from agent.subdirectory_hints import SubdirectoryHintTracker
+
+
+class TestSubdirectoryHintTrackerTildeRobustness:
+    """Regression: literal ``~`` in tool-call args must not crash the walker."""
+
+    def test_tilde_approximately_in_command_does_not_crash(self, tmp_path):
+        """LLMs use ``~`` for "approximately" (e.g. ``~500 agencies``).
+
+        ``pathlib.Path('~500-700').expanduser()`` raises ``RuntimeError`` —
+        the walker must catch this, not propagate it as a tool failure.
+        """
+        tracker = SubdirectoryHintTracker(working_dir=str(tmp_path))
+        # Heredoc-style terminal command body containing "~500-700"
+        # used as "approximately 500-700"
+        cmd = (
+            "cat > out.md <<EOF\n"
+            "Segment size signal: ~500-700 agencies in DACH region.\n"
+            "CVE volume: ~45,000 disclosed in 2025.\n"
+            "Founder blended rate: ~80/hr.\n"
+            "EOF"
+        )
+        # Must not raise — return value can be None / empty
+        tracker.check_tool_call("terminal", {"command": cmd})
+
+    def test_tilde_with_unknown_user_does_not_crash(self, tmp_path):
+        """``~unknown_user`` similarly raises RuntimeError on POSIX systems
+        whose /etc/passwd does not contain that user.  Walker must absorb it."""
+        tracker = SubdirectoryHintTracker(working_dir=str(tmp_path))
+        cmd = "echo path: ~nonexistent_user_xyzzy_12345/some/file"
+        # Must not raise
+        tracker.check_tool_call("terminal", {"command": cmd})
+
+    def test_valid_tilde_user_still_works(self, tmp_path):
+        """The fix must not regress the legitimate-tilde-user path.
+
+        ``~`` alone resolves to ``Path.home()`` and should still be
+        recognised as a candidate path (no exception either way).
+        """
+        tracker = SubdirectoryHintTracker(working_dir=str(tmp_path))
+        tracker.check_tool_call("terminal", {"command": "ls ~/Documents"})
+        # No exception, no assertion required