chore: update microsoft-foundry metadata

[qinezh]

Summary

Follow-up metadata alignment after the microsoft-foundry rename (#587) and sync (#588). Three minimal changes, all identity/branding metadata.

Changes

1. Plugin manifest description (microsoft-foundry/.github/plugin/plugin.json) -> Skills and interactive Copilot canvas for designing, configuring, testing and deploying agents to Microsoft Foundry. Aligns plugin.json with the synced extension package.json description (already on main via chore: sync microsoft-foundry (db4a09a563a2) #588's sync of db4a09a); the sync workflow never touches plugin.json, so it had the prior wording.
2. Sync workflow display name (.github/workflows/sync-microsoft-foundry.yml) -> Sync microsoft-foundry canvas (workflow name: and the job summary heading ## Sync microsoft-foundry canvas). No file rename or internal identifier changes.
3. README H1 (microsoft-foundry/README.md) -> # Microsoft Foundry Canvas.

No other plugin/canvas display names or unrelated files changed. Vendored payload untouched.

Validation

• JSON parse of plugin.json and extension package.json -> both descriptions read back byte-exact and identical.
• yaml.safe_load on the workflow -> valid; name = Sync microsoft-foundry canvas.
• Grep confirms no stale Sync Microsoft Foundry / Microsoft Foundry Sync / prior description text remains in authored files.
• Diff vs origin/main contains only the three intended files.

Commits

• f4e40a9 - chore: update microsoft-foundry plugin description and sync workflow name
• 1ba814a - docs: update microsoft-foundry README title