chore: remove dead String/IsValid methods from constants (batch 9)

[dsyme]

Removes dead String() and IsValid() methods from semantic type aliases in pkg/constants/constants.go — flagged by deadcode tool.

Deleted: LineLength.String, LineLength.IsValid, FeatureFlag.String, FeatureFlag.IsValid, URL.String, URL.IsValid, ModelName.String, ModelName.IsValid, WorkflowID.String, WorkflowID.IsValid, EngineName.String, EngineName.IsValid, MCPServerID.IsValid — 13 functions total.

[Copilot]

Pull request overview

Removes unused String() / IsValid() helper methods from several semantic type aliases in pkg/constants, along with the corresponding unit tests, as flagged by the deadcode tool.

Changes:

• Deleted String() / IsValid() methods for multiple semantic type aliases (e.g., LineLength, FeatureFlag, URL, ModelName, WorkflowID, EngineName, MCPServerID).
• Removed unit tests that exercised the deleted helper methods.

Reviewed changes

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

File	Description
pkg/constants/constants.go	Removes dead helper methods and the now-unused fmt import from constants semantic types.
pkg/constants/constants_test.go	Deletes tests that depended on the removed helper methods.

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[Copilot]

scratchpad/go-type-patterns.md includes code examples for LineLength, WorkflowID, and EngineName that still reference the deleted String() / IsValid() methods (see e.g. scratchpad/go-type-patterns.md around the “LineLength Type” / “WorkflowID Type” sections). Since this PR removes those methods, the documentation examples should be updated to avoid showing non-compiling code.

[Copilot]

Removing the exported String()/IsValid() methods from semantic types (e.g., LineLength/WorkflowID/EngineName/etc.) is a breaking change for any downstream Go code importing github.com/github/gh-aw/pkg/constants, even if the methods are unused داخل this repo. If this package is considered part of the public Go API, consider either (a) keeping these methods with // Deprecated: docs for a deprecation window, or (b) documenting the breaking change via the project’s changeset/CHANGELOG process.