feat: add CircleCI OIDC detector

[cloudsmith-iduffy]

Summary

Adds CircleCI to the OIDC credential auto-discovery chain. When running in CircleCI, the CLI reads the OIDC token from CIRCLE_OIDC_TOKEN_V2 (preferred) or CIRCLE_OIDC_TOKEN and exchanges it for a short-lived Cloudsmith API token — no static credentials required.

• New CircleCIDetector (cloudsmith_cli/core/credentials/oidc/detectors/circleci.py), registered first in the detector chain.
• Detection guards on CIRCLECI == "true" plus presence of a token env var.
• Unit tests covering detection and token selection (V2 preference, V1 fallback, error cases).
• CHANGELOG entry.

Verification

Verified end-to-end against a live public CircleCI pipeline (cloudsmith-iduffy/cloudsmith-cli-oidc-test): detection → OIDC token fetch → Cloudsmith exchange → authenticated whoami/list repos round-trip succeeds.

OIDC token claims observed:

• iss: https://oidc.circleci.com/org/<org_id>
• aud: <org_id> (the CircleCI org UUID; not customizable for the preset env-var token)
• sub (V2): org/<org_id>/project/<project_id>/user/<user_id>/vcs-origin/<vcs_origin>/vcs-ref/<vcs_ref>

Public proof pipeline: https://app.circleci.com/pipelines/github/cloudsmith-iduffy/cloudsmith-cli-oidc-test

🤖 Generated with Claude Code

[Copilot]

Pull request overview

• Adds CircleCI support to the CLI’s OIDC credential auto-discovery so CircleCI jobs can exchange a CircleCI-provided OIDC JWT for a short-lived Cloudsmith API token (avoiding static credentials).

Changes:

• Introduces a new CircleCIDetector that reads CIRCLE_OIDC_TOKEN_V2 (preferred) or CIRCLE_OIDC_TOKEN.
• Registers CircleCI first in the OIDC detector chain.
• Adds unit tests for detection/token selection and documents the feature in the changelog.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
cloudsmith_cli/core/credentials/oidc/detectors/circleci.py	Adds CircleCI environment detection and token retrieval logic for OIDC.
cloudsmith_cli/core/credentials/oidc/detectors/init.py	Registers the CircleCI detector in the environment detection chain.
cloudsmith_cli/core/tests/test_circleci_detector.py	Adds unit tests validating detection and token selection behavior.
CHANGELOG.md	Documents the new CircleCI OIDC auto-discovery capability.

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