Add a non-ActiveRecord demo resource

[tagliala]

Summary
Provide a minimal sample in response to the request from Javier Julio.

• Add a read-only API Posts resource backed by seven in-memory records, without an external API or database changes.
• Demonstrate title/status filtering, retained filter inputs, Clear Filters, and API-style pagination.
• Disable unsupported sorting, downloads, comments, batch actions, and Ransack-only active-filter chips.
• Add model, request, and browser coverage plus usage documentation.

Dependency
Temporarily pin ActiveAdmin to e372fc404d788824c0974894418313849f577a1c from activeadmin/activeadmin#9039. The released beta22 lacks the required fixes. Replace this pin once a release includes them; no monkey patches are added.

Verification

• Full Rails test suite: 28 tests passing on the latest main base.
• RuboCop lint-only checks using the installed default configuration: passing. The existing repository RuboCop runner lacks its dependency and configuration.
• Ruby syntax checks and git diff --check: passing.

AI Disclosure
This minimal implementation was produced by GPT-6 Astra and has not been reviewed or verified by a human. Automated checks do not replace human review; this PR is intentionally a draft.

Refs activeadmin/activeadmin#9039

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.10%. Comparing base (6af3731) to head (4090507).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1001      +/-   ##
==========================================
+ Coverage   94.78%   96.10%   +1.32%     
==========================================
  Files          17       19       +2     
  Lines         115      154      +39     
==========================================
+ Hits          109      148      +39     
  Misses          6        6              

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