fix(skills): stop SKILL.md frontmatter escapes compounding on every save (#5883)

_emit_scalar quotes a frontmatter scalar with json.dumps when it holds
punctuation that would change how the line reads back. _parse_scalar undid that
with a bare raw[1:-1]: it stripped the quotes but never decoded the escapes. So
a description containing ü was written as the escape sequence \u00fc, read
back with that escape still sitting literally in the value, and re-escaped on
the next save. The backslash run doubles every save, so a non-English skill
description degrades into backslash noise after a few edits, and the escapes are
shown verbatim in the skills list and the /skills catalog.

This is not limited to non-ASCII. Any description containing a quote takes the
same path, since the quote is itself what forces the quoted form.

Make the two halves symmetric: emit with ensure_ascii=False, since SKILL.md is
UTF-8 at both ends (skills.py reads it, atomic_write_text writes it) and the
ASCII-escaped form bought nothing; and parse double-quoted scalars with
json.loads, falling back to the previous literal reading when the value is not
valid JSON. Files already corrupted heal one level per load.

ensure_ascii=False on its own would open a smaller hole. json.dumps escapes
every C0 control character but passes NEL, LINE SEPARATOR and PARAGRAPH
SEPARATOR through literally, and parse_frontmatter reads one scalar per line via
str.splitlines(), which breaks on all three. Re-escape those three, and add them
plus the remaining splitlines characters to the set that forces a quoted scalar,
so none of them can reach the file bare.

Co-authored-by: Alexandre Teixeira <111787685+alteixeira20@users.noreply.github.com>

services/memory/skill_format.py

@@ -100,6 +100,18 @@ def _parse_scalar(raw: str) -> Any:
     if raw.lower() in ("null", "none", "~"):
         return None
     if (raw[0] == raw[-1]) and raw[0] in ("'", '"'):
+        if raw[0] == '"':
+            # _emit_scalar writes double-quoted scalars with json.dumps, so
+            # decode the escapes instead of only stripping the quotes. Without
+            # this, `\"` / `\\` / `\uXXXX` stayed verbatim in the value and the
+            # next save escaped their backslashes again, doubling them on every
+            # load/save cycle (issue #5210).
+            try:
+                return json.loads(raw)
+            except ValueError:
+                # Hand-written file using escapes JSON rejects (e.g. a bare
+                # Windows path). Keep the previous literal reading.
+                pass
         return raw[1:-1]
     # Try number
     try:
@@ -171,6 +183,26 @@ def parse_frontmatter(text: str) -> tuple[Dict[str, Any], str]:
     return fm, body
 
 
+# Characters that force a quoted scalar. The punctuation would otherwise change
+# how the value reads back; the second row is every character str.splitlines()
+# treats as a line break, and parse_frontmatter() reads one scalar per line, so
+# emitting one of those bare would split the value across lines.
+_FM_MUST_QUOTE = (
+    ":", "#", "[", "]", "{", "}", ",", "&", "*", "!", "|", ">", "'", '"', "%", "@",
+    "\n", "\r", "\v", "\f", "\x1c", "\x1d", "\x1e", "\x85", "\u2028", "\u2029",
+)
+
+# json.dumps escapes every C0 control character, but with ensure_ascii=False it
+# passes NEL / LINE SEPARATOR / PARAGRAPH SEPARATOR through literally, and
+# str.splitlines() still breaks on all three. Re-escape exactly those, which
+# json.loads decodes again on the way in, so the pair stays symmetric.
+_FM_POST_DUMPS_ESCAPES = (
+    ("\x85", "\\u0085"),
+    ("\u2028", "\\u2028"),
+    ("\u2029", "\\u2029"),
+)
+
+
 def _emit_scalar(v: Any) -> str:
     if v is None:
         return "null"
@@ -181,8 +213,15 @@ def _emit_scalar(v: Any) -> str:
     if isinstance(v, list):
         return "[" + ", ".join(_emit_scalar(x) for x in v) + "]"
     s = str(v)
-    if any(c in s for c in (":", "#", "\n", "[", "]", "{", "}", ",", "&", "*", "!", "|", ">", "'", '"', "%", "@")):
-        return json.dumps(s)
+    if any(c in s for c in _FM_MUST_QUOTE):
+        # ensure_ascii=False keeps non-ASCII text as itself. SKILL.md is UTF-8 at
+        # both ends (skills.py reads it, atomic_write_text writes it), so the
+        # \uXXXX form bought nothing and leaked into the parsed value (#5210).
+        out = json.dumps(s, ensure_ascii=False)
+        for ch, esc in _FM_POST_DUMPS_ESCAPES:
+            if ch in out:
+                out = out.replace(ch, esc)
+        return out
     return s