feat: add XDG Base Directory spec support for config files

[graybear-io]

Implements XDG Base Directory specification (issue #1177) with backwards compatibility for existing users.

Changes:

• Add ConfigPath(), CachePath(), StatePath() functions that respect XDG environment variables (XDG_CONFIG_HOME, XDG_CACHE_HOME, XDG_STATE_HOME)
• Default paths when XDG vars not set:
  • Config: ~/.config/circleci-cli/
  • Cache: ~/.cache/circleci-cli/
  • State: ~/.local/state/circleci-cli/
• Backwards compatibility: if legacy ~/.circleci files exist, use those instead to avoid breaking existing installations
• Update Config, UpdateCheck, and TelemetrySettings Load/Write methods to use appropriate XDG paths
• Update tests to use StatePath() for telemetry file checks

Testing:

• All existing unit tests pass
• Backwards compatibility verified with existing test suite

Fixes: #1177

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

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Note

Medium Risk
Changes where user config/telemetry/update-check data is read and written, which can affect upgrades and persistence if path resolution or legacy detection is wrong.

Overview
Implements XDG Base Directory support by splitting persisted CLI files across config, cache, and state locations via new ConfigPath(), CachePath(), and StatePath() helpers, while retaining backwards compatibility by preferring existing legacy ~/.circleci files when present.

Updates Config, UpdateCheck, and TelemetrySettings load/write paths accordingly (including defaulting FileUsed when unset), and adjusts telemetry unit tests to assert against the new state path.

^{Written by Cursor Bugbot for commit cb7a47b. This will update automatically on new commits. Configure here.}

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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TelemetrySettings may load and write to different paths

Low Severity

Unlike Config and UpdateCheck which store the file path in a FileUsed field during Load() and reuse it in WriteToDisk(), TelemetrySettings recomputes the path by calling StatePath() independently in both Load() and Write(). Since StatePath() dynamically checks whether the legacy file ~/.circleci/telemetry.yml exists, if that file is created or deleted between Load() and Write() calls, settings could be loaded from one location (e.g., XDG path) but written to another (e.g., legacy path), causing data inconsistency.