FOLLOW-UP: Add build success-path testscript (from PR #195)

[adnaan]

Context

This follow-up task was identified during the review of PR #195.

Source PR: #195
PR Title: test: add CLI testscript suite with 25 black-box tests
Suggested by: @claude[bot]

Task Description

Add a testscript that validates the happy path for tinkerdown build. Currently build_errors.txtar only tests failure cases (no input, nonexistent path). A success-path test would verify that build works end-to-end on a valid markdown file.

Challenge: The build command needs to find the tinkerdown Go module to generate build source, so the testscript environment may need special setup.

Original Comment

build success path — build_errors.txtar only tests failure cases. There's no happy-path test for a successful build run on a valid input file.

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