fix(scripts): route 'Plan template not found' per --json in setup-plan.ps1 (parity with bash)

[jawwad-ali]

Description

When no plan template resolves, the bash setup-plan.sh emits:

if $JSON_MODE; then echo "Warning: Plan template not found" >&2
else echo "Warning: Plan template not found"; fi

The PowerShell twin used Write-Warning "Plan template not found", which renders WARNING: Plan template not found on PowerShell's warning stream — diverging from bash in both wording (WARNING: vs Warning:) and routing (warning stream vs stderr/stdout per mode). It is also inconsistent with the sibling "Copied plan template" message in the same block, which #3198 just aligned to the per---json pattern.

Fix

Route the message the same way as bash and the sibling: [Console]::Error.WriteLine under -Json (so stdout stays pure JSON), Write-Output otherwise, with bash's exact wording. Completes the parity started in #3198.

Testing

• uvx ruff check clean; tests/test_ps1_encoding.py green (.ps1 stays ASCII).
• New test_ps_setup_plan_template_not_found_warning_matches_bash: with no resolvable template, -Json mode now emits Warning: Plan template not found on stderr with stdout still parseable JSON. Fails before (it was WARNING: on the warning stream), passes after.
• Verified with Windows PowerShell 5.1; existing setup-plan tests still pass.

AI Disclosure

• I did use AI assistance (describe below)

Found and fixed with Claude Code (Claude Opus 4.8) under my direction. AI spotted the leftover Write-Warning divergence beside the #3198-aligned sibling; I confirmed the bash wording/stream and verified fail-before/pass-after under Windows PowerShell 5.1, and reviewed the diff before submitting.

[Copilot]

Pull request overview

This PR aligns the PowerShell setup-plan.ps1 “plan template not found” message with the bash twin by matching both the exact wording (Warning: …) and the stream routing behavior in -Json mode (stderr so stdout remains valid JSON).

Changes:

• Replace Write-Warning "Plan template not found" with conditional output: stderr in -Json mode, stdout otherwise, using bash-matching text.
• Add a regression test ensuring -Json mode keeps stdout parseable JSON while emitting the warning on stderr when no template resolves.

Show a summary per file

File	Description
scripts/powershell/setup-plan.ps1	Routes “Warning: Plan template not found” to stderr only in -Json mode (stdout otherwise), matching bash wording and JSON-stream expectations.
tests/test_setup_plan_no_overwrite.py	Adds a PowerShell regression test asserting stderr warning + stdout JSON when the template cannot be resolved.

Review details

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• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Low

[mnriem]

Thank you!