docs: document the existing Presence API extension points

[i-am-chitti]

Description

Currently, several extension points exist within the plugin but remain undocumented in the README.md file. Furthermore, the boundary between public stable APIs and internal private helpers needs to be explicitly defined to avoid developers relying on unstable functions.

Fixes

• Document the existing Presence API extension points #13

Proposed Changes

README.md

• Added an "Extension Points" section documenting presence post type support (via register_post_type() or add_post_type_support()), the wp_presence_default_ttl filter and WP_PRESENCE_DEFAULT_TTL constant, the wp_presence_current_screen_key filter, and the wp_presence_screen_revision_bumped action.
• Cross-referenced wp_presence_current_screen_key from the "Stale-screen detection" section, so the PHP opt-in and its JS counterpart wp.presence.markScreenStale() point at each other.
• Refined the "PHP API" section to state the public/private boundary: the six listed functions are the stable contract; everything else in includes/functions.php is @access private and may change or be removed.

includes/functions.php

• Marked wp_presence_parse_room() as @access private — it was neither public nor tagged, which made the README's boundary claim inaccurate. All its callers are internal.

How to Test / Verification

Documentation change; verify the README matches the code

Usage of AI

Use: Yes
Model: Claude Code Opus 5
For: help with wordings and draft PR description

[github-actions]

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[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.13%. Comparing base (3406134) to head (0b563e8).
⚠️ Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #175   +/-   ##
=========================================
  Coverage     29.13%   29.13%           
  Complexity      164      164           
=========================================
  Files            15       15           
  Lines          1840     1840           
=========================================
  Hits            536      536           
  Misses         1304     1304           

Flag	Coverage Δ
phpunit	29.13% <ø> (ø)

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