docs: explain how the spec workflow differs from OpenSpec

[AmIrRX0]

Summary

• explain the overlap between the Common Skills spec workflow and OpenSpec
• compare their scope, artifacts, lifecycle, tooling, and customization model
• give readers practical guidance on choosing one approach or using them together without duplicate sources of truth

Validation

• checked the comparison against the current OpenSpec concepts, workflow, and team-workflow documentation
• ran git diff --check

Closes #42

[warp-for-oss]

@AmIrRX0

Every PR must be linked to a same-repo issue before Oz can review it.

This PR is linked to #42, but no linked issue is marked ready-to-implement yet. Only repository maintainers apply that label, so please wait for a maintainer to mark the issue. Once it is marked, push a new commit or comment /warp-agent-review to re-trigger review.

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[warp-for-oss]

@AmIrRX0

Every PR must be linked to a same-repo issue before Oz can review it.

This PR is linked to #42, but no linked issue is marked ready-to-implement yet. Only repository maintainers apply that label, so please wait for a maintainer to mark the issue. Once it is marked, push a new commit or comment /warp-agent-review to re-trigger review.

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