feat: add formatting helper APIs

[AchoArnold]

Summary

• add GoLand-recognizable formatting siblings for all formatting APIs
• preserve existing API behavior, caller attribution, nil propagation, and error codes
• document static versus interpolated helper usage

Validation

• go test -cover ./...
• go vet ./...
• golangci-lint fmt --diff
• golangci-lint run
• pre-commit run --all-files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (209fcdb) to head (c8eab77).

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #3   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            4         4           
  Lines          137       153   +16     
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+ Hits           137       153   +16     

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[Copilot]

Pull request overview

Adds GoLand-recognizable *f formatting helper siblings to the stacktrace public API, while aiming to preserve existing behavior around caller attribution (runtime.Caller(2)), nil propagation, and error-code semantics.

Changes:

• Added five new exported helper functions: NewErrorf, Propagatef, NewErrorWithCodef, PropagateWithCodef, NewMessageWithCodef.
• Added black-box tests to validate formatting output, code inheritance/override, nil propagation, and caller attribution for the new helpers.
• Updated README examples/signatures to use the *f helpers for interpolated messages, and added design/plan docs describing the API intent.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
stacktrace.go	Adds the new *f formatting helper APIs alongside existing constructors/wrappers.
stacktrace_test.go	Adds coverage for formatting helpers’ output, code behavior, nil propagation, and caller capture.
functions_for_test.go	Adds call-site fixtures used to validate caller attribution in tests.
README.md	Documents *f helpers and updates interpolated examples/signatures.
docs/superpowers/specs/2026-07-19-formatting-helper-api-design.md	Design spec for the formatting helper API addition.
docs/superpowers/plans/2026-07-19-formatting-helper-api.md	Implementation plan documenting steps/tests for the change.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

README.md:146

• The %w unsupported-behavior caveat is currently in a [!NOTE] block, but it's an important behavioral difference from fmt.Errorf that can easily surprise users. Consider using [!IMPORTANT] here (consistent with other high-signal caveats in this README).

> [!NOTE]
> Unlike `fmt.Errorf`, `%w` is not supported; use `Propagate` or `Propagatef` to
> wrap a cause.