[M1nt-Ch0c0]

Result / 用户结果

With the welcome screen enabled, a WSL or Unix pane using fish receives a command fish can execute instead of a POSIX if ... then ... fi expression left at its prompt. Closes #803.

Design / 设计边界

• Keep the existing welcome-command authority and run the POSIX body through sh -c. The outer invocation is valid in fish, bash and sh; Windows PowerShell behavior is unchanged.
• Preserve the fastfetch branch and the Pebrel/uname fallback. No new dependencies, persistence, or lifecycle changes.

Evidence / 验证依据

• WSL Arch Linux, fish 4.9.3: the original command fails fish -n -c; the new command passed actual sh, bash and fish execution with and without fastfetch. An interactive fish PTY executed the new command and accepted subsequent input.
• The added Rust regression test passed in an isolated rustc --test harness extracted from the production function and test (the harness supplies a temporary-directory helper). It exercises the real shell commands; fish and bash are skipped when absent.
• python scripts/check_architecture.py --base origin/main: passed.
• python -m unittest scripts.tests.test_architecture_budgets scripts.tests.test_architecture_dependencies scripts.tests.test_architecture_governance: 55 passed.
• git diff --check: passed. Full Cargo product build and project test suite were not run: the pinned Rust 1.97.1 toolchain was not available locally. CI must verify the full product and native platforms.

Required Review / 必须确认

• I followed CONTRIBUTING.md, docs/architecture.md, and docs/project-constraints.md.
• I split responsibilities, not arbitrary line ranges; no duplicate behavior authority was added.
• python scripts/check_architecture.py --base origin/main passes; budgets were not inflated to fit the change.
• Tests cover success and failure; platform/feature coverage limitations are stated.
• New messages use typed i18n IDs and matching placeholders; untranslated content has an explicit fallback. (Not applicable: no new UI text.)
• Governance changes include a counterexample, corrected contract, tests, and a maintainer-reviewed decision. (Not applicable.)