CI: Update change detection

[losolio]

This removes our dependency on an external GitHub Action for change detection and keeps the workflow logic self-contained in the repository. The main goal is to reduce supply-chain exposure.

[Copilot]

Pull request overview

This PR replaces the third-party dorny/paths-filter GitHub Action with a self-contained shell script for detecting changed file paths, reducing supply-chain exposure. The new script uses git diff and grep to determine which subdirectories have changed and sets corresponding outputs that gate downstream CI jobs.

Changes:

• Replaced dorny/paths-filter@v3 with a custom bash script that uses git diff --name-only and grep to detect changes across api, web, and infra paths.
• Pinned actions/checkout to a specific commit SHA (claimed v6.0.2) with fetch-depth: 0 to support full history diffing.
• Updated step and output references from steps.filter.outputs.* to steps.check.outputs.*.

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