Add repository-specific AGENTS.md guidance for Copilot workflows

[Copilot]

This PR adds a root-level AGENTS.md to give Copilot/agent sessions deterministic, repo-specific operating guidance. It documents how to work in this Astro blog codebase without relying on implicit conventions.

• Scope

  • Adds /AGENTS.md only (no source/runtime changes).

• Repository operating model

  • Captures key structure for agent navigation:
    • blog content location (/src/content/blog)
    • content schema location (/src/content.config.ts)
    • utility/test location (/src/utils)

• Execution and tooling contract

  • Defines the expected runtime/toolchain for agent sessions:
    • Node 22
    • pnpm + lockfile-driven install
  • Includes canonical setup and command surface for day-to-day tasks.

• Change and validation expectations

  • Documents minimal-change guardrails (e.g., avoid touching generated /dist unless requested).
  • Specifies the validation command set agents should run and aligns it with CI workflow expectations.

## Validation before finishing

```bash
pnpm test
pnpm run check
pnpm run build

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by firewall rules:
>
> - `telemetry.astro.build`
>   - Triggering command: `/home/REDACTED/work/_temp/ghcca-node/node/bin/node node /home/REDACTED/work/mahata.github.io/mahata.github.io/node_modules/.bin/../astro/bin/astro.mjs build` (dns block)
>
> If you need me to access, download, or install something from one of these locations, you can either:
>
> - Configure [Actions setup steps](https://gh.io/copilot/actions-setup-steps) to set up my environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this repository's [Copilot coding agent settings](https://github.com/mahata/mahata.github.io/settings/copilot/coding_agent) (admins only)
>
> </details>