SmartJoin incorrectly treats relative paths containing special variable names as special paths

[znnnnnnn-wil]

Fixes #3021

Description

SmartJoin incorrectly treats ordinary relative paths containing special variable names such as .ROOT_DIR as absolute/special paths, causing them to bypass the base-directory join.

The path should be treated as a normal relative path and joined with base.

From looking at the implementation, this may be caused by isSpecialDir using strings.Contains to detect special variables. As a result, an ordinary directory name containing one of these strings can be matched even when the variable is not actually part of a {{ ... }} template expression.

A small portion of the code and tests was generated with assistance from Codex.

Linux amd64 / Go 1.26.5
go test ./...                                      PASS

golangci-lint 2.13.0
fmt --diff (3 changed files)                       PASS
run --new-from-rev=385e5ad --timeout 5m             0 issues
run --timeout 5m                                  3 pre-existing gci issues:
  errors/error_taskfile_decode.go
  errors/errors.go
  errors/errors_task.go

Checklist

• I have read and followed the Contribution Guide.
• I have disclosed the use of any AI-generated content in this pull request per the AI Usage Policy.
• I fully understand the changes and have hand-written the description (No AI) of this pull request.

[trulede]

Um, AI wrote the code, correct?
The PR will get rejected in its current state.

[znnnnnnn-wil]

Yes, Codex helped with part of the implementation and tests.

I worked through the issue and the scope of the fix myself, and went over the implementation a few times before opening the PR. I also changed some parts manually after reviewing the generated code.

I'm familiar with the behavior this is trying to preserve — in particular, project.ROOT_DIR/... should still be treated as a normal relative path, while something like {{.ROOT_DIR}}/... should keep the existing special-path behavior.

I ran the tests and lint locally and checked the regression cases before submitting.

Happy to explain any part of the change or adjust it based on feedback.

[trulede]

Why wouldn't you just do this?

var knownAbsDirs = []string{
	"{{.ROOT_DIR}}",
	"{{.TASKFILE_DIR}}",
	"{{.USER_WORKING_DIR}}",
}