fix(desktop): quote persisted @image: paths so spaced paths render

The unquoted alternative in the directive pattern is `\S+`, so a ref built
by string interpolation truncates at the first space and strands the tail
as loose text next to a broken thumbnail. Composer images live in the app's
userData dir, which on macOS is `~/Library/Application Support/<App>/` — so
every pasted or dropped image hit this.

Adds format_reference_value next to REFERENCE_PATTERN, mirroring
formatRefValue in the desktop's directive-text.tsx, and covers the
round-trip through the parser.

agent/context_references.py

@@ -19,6 +19,7 @@ REFERENCE_PATTERN = re.compile(
     rf"(?<![\w/])@(?:(?P<simple>diff|staged)\b|(?P<kind>file|folder|git|url):(?P<value>{_QUOTED_REFERENCE_VALUE}(?::\d+(?:-\d+)?)?|\S+))"
 )
 TRAILING_PUNCTUATION = ",.;!?"
+_NEEDS_QUOTING = re.compile(r"""[\s()\[\]{}<>"'`]""")
 _SENSITIVE_HOME_DIRS = (".ssh", ".aws", ".gnupg", ".kube", ".docker", ".azure", ".config/gh")
 _SENSITIVE_HERMES_DIRS = (Path("skills") / ".hub",)
 _SENSITIVE_HOME_FILES = (
@@ -60,6 +61,21 @@ class ContextReferenceResult:
     blocked: bool = False
 
 
+def format_reference_value(value: str) -> str:
+    """Quote a reference value so ``REFERENCE_PATTERN`` reads it back whole.
+
+    The unquoted alternative in the pattern is ``\\S+``, so a path containing a
+    space parses as a truncated ref with the tail left behind as loose text.
+    Mirrors ``formatRefValue`` in the desktop's directive-text.tsx.
+    """
+    if not _NEEDS_QUOTING.search(value):
+        return value
+    for quote in ("`", '"', "'"):
+        if quote not in value:
+            return f"{quote}{value}{quote}"
+    return value
+
+
 def parse_context_references(message: str) -> list[ContextReference]:
     refs: list[ContextReference] = []
     if not message: