fix(ci): Use correct organization secret name CARGO_TOKEN

[konard]

Summary

This PR fixes the crates.io publishing failure caused by a mismatch between the organization secret name and the workflow's secret reference.

Problem

After PR #32 added the cargo publish step, the release still failed to publish to crates.io with the error:

error: failed to publish browser-commander v0.4.0 to registry at https://crates.io

Caused by:
  please provide a non-empty token

Root Cause: The workflow referenced secrets.CARGO_REGISTRY_TOKEN but the organization secret is named CARGO_TOKEN. Since there's no secret named CARGO_REGISTRY_TOKEN, GitHub returned an empty string.

Solution

Map the organization secret CARGO_TOKEN to the environment variable CARGO_REGISTRY_TOKEN:

env:
  CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_TOKEN }}

This ensures:

1. The workflow uses the existing organization secret
2. The environment variable name matches what Cargo expects

Changes

• .github/workflows/rust.yml: Updated secret reference from secrets.CARGO_REGISTRY_TOKEN to secrets.CARGO_TOKEN in both auto-release and manual-release jobs
• docs/case-studies/issue-33/: Added comprehensive case study with timeline, root cause analysis, and lessons learned

Test Plan

• YAML syntax validated
• CI run after merge should show CARGO_REGISTRY_TOKEN: *** (masked value)
• No warning about missing token should appear
• Package should be published to crates.io

References

• Case study: docs/case-studies/issue-33/README.md
• CI Run showing the problem: https://github.com/link-foundation/browser-commander/actions/runs/21116038007
• Cargo Registry Documentation: https://doc.rust-lang.org/cargo/reference/registries.html

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Fixes #33

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[konard]

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