chore: add update-dependencies.sh automation script

[josecelano]

Summary

Add a reusable shell script scripts/update-dependencies.sh that automates the dependency update workflow:

1. Ensures a clean working tree
2. Fetches and fast-forwards the base branch
3. Creates a feature branch
4. Runs cargo update and captures the full output
5. Optionally runs ./scripts/pre-commit.sh
6. Commits the Cargo.lock changes with the full cargo update output in the commit body
7. Pushes the branch to the fork remote
8. Optionally creates a PR via gh pr create

Commit signing is on by default (git commit -S) and can be disabled with --no-sign-commit.

Usage

./scripts/update-dependencies.sh \
  --branch 445-update-dependencies \
  --push-remote josecelano \
  --create-pr

See ./scripts/update-dependencies.sh --help for all options.

[josecelano]

Implemented via #447