Add SharePoint transport

[llbbl]

Closes #4

Summary

• Add a SharePoint transport with app-only MSAL client-credential auth using the Microsoft Graph .default scope.
• Upload and replace DOCX files through the Graph simple-upload endpoint while returning stable drive item metadata.
• Document SharePoint-specific path rules, permissions, and testing coverage for the TeamWiki-style consumer flow.

Changes

Features

• src/sharepoint.ts: Add SharePoint transport, token-provider helpers, upload flow, path validation, and sanitized typed errors.
• src/index.ts: Export the new SharePoint transport surface.
• src/transport.ts: Add SharePoint transport error codes.

Tests

• tests/index.test.ts: Add mocked Graph coverage for auth, upload, overwrite/version-history behavior, path validation, and acceptance-style flows.

Documentation

• README.md: Document SharePoint transport setup, .default auth, Sites.Selected plus site-specific write grants, and behavior limits.
• package.json: Add the MSAL dependency.
• pnpm-lock.yaml: Lock the MSAL dependency graph.
• .gitignore: Ignore additional local context artifacts.

Test plan

• pnpm install --frozen-lockfile
• pnpm check (116 passed, 2 skipped)
• Forced real-Pandoc Vitest run (117 passed, 1 platform skip)
• Node 20, 22, and 24 matrix checks
• actionlint
• Package contents check
• Independent JS/TS review PASS
• Live Microsoft tenant integration test (not run by design)

Notes

• App-only auth uses the Graph .default scope; Sites.Selected remains the application permission plus site-specific grant, not an OAuth scope.
• Uploads use Graph simple PUT, so upload sessions for files larger than 250 MB are not included in this change.