chore: refactor into a separate /dues page

[davidjenni]

• fix: npm audit detects 5 severe vulnerabilities

• chore: add AGENTS.md for agents in VSCode CoPilot, opencode, pi.dev agent, etc.
  This change is optional, I'm fine to remove this commit if this is contentious

• chore: refactor into a separate dues page for a separate /dues slug:

The PayPal/check payment section in /about is moved into its separate /dues page.
This allows to point members e.g. in emails via this slug https://microhams.com/dues

Look of header:

[Copilot]

Pull request overview

This PR introduces a dedicated /dues landing page sourced from the pages content collection, updates site navigation and About content to reference it, and refreshes dependencies to address npm audit vulnerabilities.

Changes:

• Added /dues route (src/pages/dues.astro) backed by a new markdown content entry (src/content/pages/dues.md).
• Updated /about content to remove embedded dues/payment details and link to the new dues page.
• Added a “Dues” item to the global header navigation and updated package-lock.json with dependency security upgrades.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/pages/dues.astro	New route that renders the pages/dues content entry via MarkdownLayout.
src/content/pages/dues.md	New dues/payment instructions content including PayPal form and mailing address.
src/content/pages/about.md	Removes dues/payment block and replaces it with a link to /dues; formatting tweaks.
src/components/SiteHeader.astro	Adds “Dues” link in primary nav.
package-lock.json	Updates transitive dependencies consistent with npm audit remediation.
AGENTS.md	Adds repository overview/reference for agent tooling.

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