Add GitHub Action to update nonce query parameter on issue trigger

[Copilot]

Closes: #[issue_number]

Implements automated cache-busting for README images via GitHub Action that triggers on issue open events containing "nonce" keyword.

Implementation

• Workflow trigger: Issues with "nonce" in body (case-insensitive)
• Pattern matching: Targets nonce=NNNNNN (6 digits) within <!-- NONCE --> section markers only
• Replacement logic: Generates pseudo-random 6-digit value via printf "%06d" $((RANDOM % 1000000)), replaces all matches globally with same value
• Security: Issue body passed through environment variables to prevent command injection; explicit GITHUB_TOKEN permissions (contents: write, issues: read)

Example

When an issue containing "nonce" is opened, both URLs in the NONCE section update atomically:

<!-- NONCE -->
[![card](https://example.com/api/pin?nonce=123456)](...)
[![card](https://example.com/api/pin?nonce=123456)](...)
<!-- NONCE-END -->

↓

<!-- NONCE -->
[![card](https://example.com/api/pin?nonce=047829)](...)
[![card](https://example.com/api/pin?nonce=047829)](...)
<!-- NONCE-END -->

The BASELINE section (without nonce parameter) remains unchanged.

Original prompt

create a GitHub Action that is triggered when an Issue is open - if the Issue body contains the word "nonce", then it should continue, else stop - if it continues it should scan the body of the README.md for the section delimited by HTML comments NONCE and NONCE-END - within that body it should apply pattern matching to identify any text of the form "nonce=NNNNNN" ("nonce", followed by "=", followed by 6 digits) - it should generate a pseudo-random 6-digit numeral - and then replace the value of NNNNNN with that numeral (same numeral globally in the readme, i.e., if the pattern matches more than once, the value should be the same

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[hesreallyhim]

@copilot is it posssible to throttle the action so that it will not return if an existing run of that same workflow is in progress? we should also have a visual indicator that shows the timestamp that the nonce was last changed.