feat: label agent presence rows across the admin UI

[i-am-chitti]

Fixes #549

An agent editing over REST/MCP runs no Heartbeat, so nothing showed it was there. It already writes its own presence row with a capped $expires_in in the post room it's editing - wp_presence_is_agent_user() (deferred to wpai_is_agent_user() when the Agent Users plugin is loaded) labels that row, and wp_presence_admin_room_entries() backfills it into admin/online reads so the admin bar picks it up without a second write.

Labelled: the admin bar dropdown (incl. crowd tooltips), the Active Posts widget, and the post list's Editors column. Left out of the wp_presence_collaboration_started count on purpose - that only watches editor- prefixed rows.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): claude-sonnet-5
Used for: Implementation, tests, docs. Reviewed by me.

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Codecov Report

❌ Patch coverage is 92.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.99%. Comparing base (a8b869e) to head (d7395c9).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #653      +/-   ##
============================================
- Coverage     95.24%   94.99%   -0.25%     
- Complexity      195      199       +4     
============================================
  Files            24       24              
  Lines          3257     3298      +41     
============================================
+ Hits           3102     3133      +31     
- Misses          155      165      +10     

Flag	Coverage Δ
multisite	94.99% <92.00%> (-0.25%)	⬇️
phpunit	70.10% <92.00%> (+0.06%)	⬆️

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