* fix(mcpb): Remove unsupported 'skills' key from manifest (#389)

Claude Desktop rejects manifests with unrecognized keys. The 'skills' key
is not part of the current MCPB schema, causing extension preview to fail.

Skills are distributed separately via the VS Code extension and manual
installation - they don't need to be in the MCPB manifest.

Fixes #389

* docs: clarify Claude Desktop vs Claude Code skills support

* docs: update SKILL.md files with proper structure and official citations

- Recreated skills/excel-mcp/SKILL.md and skills/excel-cli/SKILL.md with:
  - Official 'compatibility' field instead of markdown warning boxes
  - Merged behavioral-rules.md content into main SKILL files
  - Proper @references links to references/ directory
  - Added disable-model-invocation, license, version, repository, documentation fields

- Updated docs/AGENT-SKILLS-RESEARCH.md with official source citations:
  - agentskills.io specification
  - code.claude.com/docs/en/skills
  - VS Code Copilot agent skills documentation
  - vercel-labs/skills repository patterns

- Standardized all documentation to use PowerShell code blocks (not bash)
- Fixed inconsistent formatting across 27+ documentation files

* fix(release): correct agent skill filenames in release notes

Build script creates excel-*-skill-v{version}.zip (with v prefix),
but release notes referenced files without the v prefix.

* fix(docs): correct agent skill filename and paths in INSTALLATION.md

- Changed non-existent excel-mcp-skills-{version}.zip to excel-cli-skill-v{version}.zip
- Updated skill directory paths from excel-mcp to excel-cli (matches CLI skill package)

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Co-authored-by: Stefan Broenner <stefan.broenner@microsoft.comm>