feat: add convert-excel-to-md, convert-pdf-to-md, and convert-word-to-md skills (#2294)

* feat: add convert-excel-to-md, convert-pdf-to-md, and convert-word-to-md skills

Add three new agent skills that convert common document formats to Markdown
using bundled Python scripts powered by MarkItDown:

- convert-excel-to-md: Converts .xlsx workbooks to Markdown with per-sheet
  tables and embedded image extraction via a bundled Python script.
- convert-pdf-to-md: Converts .pdf documents to Markdown with text/table
  extraction and embedded image extraction via PyMuPDF.
- convert-word-to-md: Converts .docx documents to Markdown with proper
  image extraction replacing MarkItDown's base64 placeholders.

Each skill includes:
- SKILL.md with detailed usage instructions, output structure docs, and
  a troubleshooting table
- scripts/ with the conversion Python script and requirements.txt
- references/setup.md with environment setup instructions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: remove Markdown image syntax from convert-word-to-md SKILL.md

The CI valid-refs linter flagged the literal Markdown image syntax
containing a data URI as an invalid file reference. Replaced it with
a plain text description of the placeholder format.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: update pip install command to use scripts/requirements.txt for setup

* fix: clarify installation instructions for requirements-file in setup documentation

* fix: correct indentation in image extraction function

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* feat: add comprehensive documentation and setup instructions for convert-to-md skills

* fix: add trailing slashes to skill paths in plugin.json

* fix: reorder skills in plugin.json for consistency

* fix: update plugin.json and README.md for clarity and consistency

* feat: add convert-to-md plugin and update related documentation

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* fix: enhance conversion skills to handle mixed file types and improve error handling

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

.github/plugin/marketplace.json

@@ -253,6 +253,12 @@
       "description": "Coding agents hallucinate APIs. ContextMatic gives them curated, versioned API and SDK docs. Ask your agent to \"integrate the payments API\" and it guesses — falling back on outdated training data and generic patterns that don't match your actual SDK. ContextMatic solves this by giving the agent deterministic, version-aware, SDK-native context at the exact moment it's needed.",
       "version": "0.1.0"
     },
+    {
+      "name": "convert-to-md",
+      "source": "plugins/convert-to-md",
+      "description": "A collection of Copilot skills that convert common document formats into Markdown so their contents can be accurately analyzed, summarized, searched, or extracted from. Just tell Copilot what you need — the right skill is invoked automatically and the conversion happens behind the scenes.",
+      "version": "1.0.1"
+    },
     {
       "name": "copilot-goal-skill",
       "description": "Goal-driven task orchestration with independent verification. Interviews the user to define a clear goal, then loops between a Builder subagent (does the work) and an Inspector subagent (judges the result with fresh context). The Inspector never trusts the Builder. Output is auditable in git commits from each subagent actions. Use when the user says \"achieve this goal\", \"make this work\", \"implement until done\", or wants verified autonomous task completion with independent quality review.",