pptx: Add surgical mode to replace.py (safer default for partial edits)

[spm1001]

Problem

replace.py clears ALL text shapes in a presentation, then only restores those with "paragraphs" defined in the replacement JSON. This is a footgun for partial edits.

Example failure:

• User wants to edit only slide 1 (2 shapes)
• Creates JSON specifying only slide-1
• Script clears all 67 shapes across 26 slides
• Only restores 2 shapes
• Result: 64 shapes wiped, presentation destroyed

While this behavior is documented, it's surprising and destructive for the common use case of making targeted edits to specific slides.

Solution

Add surgical mode (now the default) that only touches shapes explicitly listed in the replacement JSON. The old "clear everything" behavior remains available via --clear-all flag for template workflows where starting fresh is desired.

# Surgical mode (default) - safe for partial edits
python scripts/replace.py working.pptx replacement-text.json output.pptx

# Clear-all mode - for template workflows
python scripts/replace.py working.pptx replacement-text.json output.pptx --clear-all

Changes

replace.py:

• Add --clear-all flag (surgical mode is default)
• In surgical mode, skip shapes without "paragraphs" in replacement JSON
• Add shapes_skipped counter to stats output
• Use argparse for cleaner CLI handling
• Update docstring to explain both modes

SKILL.md:

• Document surgical mode as default behavior
• Document --clear-all flag for template workflows
• Update examples to show both modes

Backward Compatibility

This is a breaking change in default behavior:

• Before: All shapes cleared by default
• After: Only specified shapes modified by default

Users who rely on the old behavior for template workflows can add --clear-all to preserve it. This seems like the right trade-off since partial edits are likely more common than full template replacements, and the old default was destructive.