fix(ci): restore Cypress binary setup for e2e runs

[fpigeonjr]

Summary

• cache the Cypress binary in GitHub Actions and install/verify it before e2e and a11y Cypress runs
• prevent fresh CI runners from failing when the npm package exists but the Cypress binary is missing from the runner cache
• switch the local TruffleHog pre-commit hook to the proven changed-file filesystem scan so commits work cleanly from git worktrees

Testing

• ran bunx cypress install && bunx cypress verify in frontend/
• ran pre-commit run trufflehog --files .pre-commit-config.yaml .github/workflows/ci.yml .github/workflows/e2e_test_reusable.yml

[Copilot]

Pull request overview

This PR updates CI and local developer tooling to make Cypress-based E2E/a11y runs more reliable on fresh runners and to improve the TruffleHog pre-commit experience (notably for git worktrees).

Changes:

• Cache the Cypress binary in GitHub Actions and run cypress install/verify before E2E and a11y Cypress runs.
• Update the local TruffleHog pre-commit hook to scan changed files via trufflehog filesystem (instead of a git-based scan).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
.pre-commit-config.yaml	Changes TruffleHog local hook entry to compute changed files and scan via filesystem.
.github/workflows/e2e_test_reusable.yml	Adds Cypress binary caching + install/verify prior to running E2E specs.
.github/workflows/ci.yml	Adds Cypress binary caching + install/verify prior to running the a11y regression Cypress specs.

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