chore(openapi): rename Unstable API version to Preview

[fern-support]

Description

Intercom has renamed their "Unstable" API version to "Preview." The change is backwards-compatible on Intercom's side (Unstable still works), but all references in the spec, SDK config, and docs should now use the new name going forward.

Changes Made

• OpenAPI spec (descriptions/0/): Updated info.version, intercom_version enum, schema refs (intercom_version_unstable → intercom_version_preview), and all code examples to use "Preview"
• Stable spec (descriptions/2.11/): Updated intercom_version enum entry from Unstable to Preview
• Fern config: Renamed unstable-openapi-overrides.yml → preview-openapi-overrides.yml, updated namespace from unstable to preview, and updated x-fern-version enum in stable overrides
• Postman collection: Renamed postman/Unstable/ → postman/Preview/ directory and updated all Intercom-Version header values
• Docs & skills: Updated CLAUDE.md, PR template, and generate-openapi-from-pr skill to reflect the new naming

Testing

• Verified all references to "Unstable" / "unstable" have been updated to "Preview" / "preview"
• No functional changes — this is a pure rename