Add --local flag to login and ci commands

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Problem

hookdeck login and hookdeck ci lacked a --local flag, making it impossible to save directory-specific credentials to .hookdeck/config.toml without the less-discoverable global --config workaround. This was inconsistent with hookdeck project use --local.

Solution

Added --local flag to both login and ci commands following the same pattern as project use --local:

• After a successful login, if --local is set, credentials (API key, project ID, project mode) are written to .hookdeck/config.toml in the current directory via the existing Config.UseProjectLocal() method.
• The .hookdeck/ directory is created if it does not exist.
• A security warning is printed when the file is newly created, advising users to add .hookdeck/ to .gitignore.
• Using --local together with --config returns a clear error.
• A shared saveLocalConfig() helper in login.go is reused by both commands to avoid duplication.