fix(cli): catch OSError in _resolve_attachment_path to prevent ENAMETOOLONG dropping long slash commands

When the user pastes a long slash command like \`/goal <long prose>\` into
\`hermes chat\`, the input flows into \`_detect_file_drop()\`, whose
\`starts_like_path\` prefilter accepts anything starting with \`/\` and
forwards it to \`_resolve_attachment_path()\`. That helper calls
\`Path.exists()\` which invokes \`os.stat()\`, which raises
\`OSError(errno=ENAMETOOLONG)\` — 63 on macOS, 36 on Linux — when the
candidate exceeds NAME_MAX (typically 255 bytes).

The OSError propagates up to the broad \`except Exception\` in
\`process_loop\` (cli.py:11798), gets logged at WARNING level, and the
user's input is silently dropped. From the user's POV the chat prompt
hangs — the only signal is in agent.log:

  WARNING cli: process_loop unhandled error (msg may be lost):
    [Errno 63] File name too long: "/goal Drive the space board..."

This affects any slash command with prose-length arguments — \`/goal\`
in particular but also \`/skill\`, \`/cron\`, custom user commands.

Fix: wrap the \`exists()\`/\`is_file()\` calls in try/except OSError so
structurally-invalid path candidates cleanly return None. The slash-
command dispatch path downstream (cli.py:11718) then handles the
input correctly.

Tests: two new regression cases in test_cli_file_drop.py cover the
original \`/goal\` reproducer and a synthetic long path. All 35 file-
drop tests pass.

Reproducer (without the fix):
  python -c "from cli import _detect_file_drop;
             _detect_file_drop('/goal ' + 'a'*300)"
  → OSError: [Errno 63] File name too long

cli.py

@@ -1550,7 +1550,21 @@ def _resolve_attachment_path(raw_path: str) -> Path | None:
     except Exception:
         resolved = path
 
-    if not resolved.exists() or not resolved.is_file():
+    # Path.exists() / is_file() invoke os.stat(), which raises OSError when
+    # the candidate string is structurally invalid as a path — most commonly
+    # ENAMETOOLONG (errno 63 on macOS, errno 36 on Linux) when the input
+    # exceeds NAME_MAX (typically 255 bytes). This bites pasted slash
+    # commands like `/goal <long prose>` because `_detect_file_drop()`'s
+    # `starts_like_path` prefilter accepts any input starting with `/`,
+    # then this resolver tries to stat it before short-circuiting on the
+    # slash-command path. Without this guard the OSError propagates up to
+    # the process_loop catch-all in _interactive_loop and the user input
+    # is silently lost (the warning ends up in agent.log but the user sees
+    # nothing — the prompt just hangs).
+    try:
+        if not resolved.exists() or not resolved.is_file():
+            return None
+    except OSError:
         return None
     return resolved