Fix _add_path_candidate crash on ~user expanduser() RuntimeError

[gtyler]

Problem

_add_path_candidate in agent/subdirectory_hints.py calls Path(raw_path).expanduser() inside try/except (OSError, ValueError). But Path.expanduser() raises RuntimeError("Could not determine home directory.") — not OSError/ValueError — for a ~user token whose user is not a real account (no /etc/passwd entry), even when $HOME is set (the ~user form does a passwd lookup, not a $HOME substitution).

Because RuntimeError isn't caught, it escapes _extract_paths_from_command → check_tool_call → execute_tool_calls_sequential → _execute_tool_calls → run_conversation and aborts the whole agent turn. On a long-running gateway this fires whenever a model emits a path-like ~name/... token for a non-existent user (~3×/24h in our deployment; each one kills the turn, e.g. an aborted scheduled run).

Reproduce

from pathlib import Path
Path("~nonexistent_user/x").expanduser()   # RuntimeError: Could not determine home directory.

Fix

The directory-hint walk is best-effort, so broaden the except to include RuntimeError and skip the bad path instead of crashing. Only the _add_path_candidate block is changed — the two structurally-identical except (OSError, ValueError) blocks in _is_valid_subdir / _load_hints_for_directory operate on already-resolved Paths (no expanduser) and are left untouched.

Fixes #43963

[alt-glitch]

Duplicate of #29433 — that open PR adds RuntimeError to the except clauses in agent/subdirectory_hints.py for the same Path.expanduser() crash on ~user/tilde tokens. Related: #41870 (shared safe_expanduser() sweep).

[gtyler]

Closing as a duplicate of the earlier #29433, which makes the same RuntimeError fix to _add_path_candidate. Deferring to it — added a comment there noting the second call site (hermes_cli/kanban_db.py:_is_managed_scratch_path) and a regression-test suggestion. Repro/diagnosis tracked in #43963.