Update docx, pptx, and xlsx skills (#1447)

Add support for template formats (.dotx, .potx, .xltx) across validation
and the helper scripts.

Consolidate the shared office helpers into a single module, replacing the
pack/unpack pipeline with explicit zip/unzip steps. Extraction now rejects
symlink and path-traversal archive entries. Move run merging to a
standalone docx script, since it only ever applied to Word documents.

Fix the redlining validator so it compares against the original even when
a document has no tracked changes, which is when an untracked edit would
otherwise go unreported.

Provision a LibreOffice user profile per invocation so conversions work in
sandboxed environments.

Trim the skill docs to the guidance that earns its place.

skills/pptx/scripts/office/helpers/pptx_slide.py

@@ -0,0 +1,60 @@
+"""Pick the slide-XML schema errors PowerPoint refuses the file over.
+
+A denylist over lxml's messages, so an unrecognised error class is a miss rather
+than a false alarm.
+"""
+
+
+from __future__ import annotations
+
+import re
+
+SLIDE_PART_RE = re.compile(
+    r"ppt/(slides|slideLayouts|slideMasters|notesSlides|notesMasters|handoutMasters)"
+    r"/[^/]+\.xml"
+)
+
+FATAL_SLIDE_ERRORS: tuple[tuple[re.Pattern[str], str], ...] = (
+    (
+        re.compile(r"\}tableStyleId': This element is not expected"),
+        "two <a:tableStyleId> in one <a:tblPr> (the schema allows one)",
+    ),
+    (
+        re.compile(r"\}srgbClr', attribute 'val'"),
+        "a colour that is not six hex digits",
+    ),
+    (
+        re.compile(r"\}txBody': Missing child element"),
+        "a <p:txBody> with no children",
+    ),
+    (
+        re.compile(r"\}miter', attribute 'lim'"),
+        'a line join with lim="NaN"',
+    ),
+    (
+        re.compile(r"\}uLnTx': This element is not expected"),
+        "<a:uLnTx> in a position the schema forbids",
+    ),
+    (
+        re.compile(r"\}overrideClrMapping': This element is not expected"),
+        "<p:overrideClrMapping> in a position the schema forbids",
+    ),
+    (
+        re.compile(r"\}nvGrpSpPr': Missing child element"),
+        "a <p:nvGrpSpPr> with no children",
+    ),
+)
+
+
+def is_schema_verdict(error: str) -> bool:
+    return error.startswith("Element ")
+
+
+def fatal_slide_errors(errors: set[str]) -> list[str]:
+    out = []
+    for error in sorted(errors):
+        for pattern, meaning in FATAL_SLIDE_ERRORS:
+            if pattern.search(error):
+                out.append(f"{meaning}: {error}")
+                break
+    return out