feat: add policy content, exporter and eraser for presence data

[josephfusco]

Presence is personal data, and core's privacy tools were wired to none of it.

Fixes #393
Related: #400

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Assisting with the implementation and tests

[github-actions]

🛝 WordPress Playgrounds

Built from 7cbef61

	5 users	40 users
Single site
Multisite

[codecov]

Codecov Report

❌ Patch coverage is 78.65169% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.84%. Comparing base (2e2ed91) to head (7cbef61).

❌ Your patch check has failed because the patch coverage (78.65%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #449      +/-   ##
============================================
- Coverage     96.37%   95.84%   -0.53%     
  Complexity      255      255              
============================================
  Files            20       21       +1     
  Lines          2923     3012      +89     
============================================
+ Hits           2817     2887      +70     
- Misses          106      125      +19     

Flag	Coverage Δ
multisite	95.84% <78.65%> (-0.53%)	⬇️
phpunit	71.11% <78.65%> (+0.22%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props joefusco.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.