fix(scripts): emit 'Copied plan template' status in setup-plan.ps1 (parity with bash)

[jawwad-ali]

Description

setup-plan.sh prints a status message after copying the plan template on a fresh run:

cp "$TEMPLATE" "$IMPL_PLAN"
if $JSON_MODE; then echo "Copied plan template to $IMPL_PLAN" >&2
else echo "Copied plan template to $IMPL_PLAN"; fi

The PowerShell twin setup-plan.ps1 emitted nothing on the successful-copy path — only the Plan already exists skip message and the Plan template not found warning had equivalents. So on a first run the two scripts had a divergent status-output contract (the bash behavior is even covered by a test, test_setup_plan_json_parseable_on_first_run, which asserts Copied plan template on stderr).

Fix

Emit the same message after WriteAllText, routed exactly like the sibling Plan already exists message — [Console]::Error.WriteLine in -Json mode so stdout stays pure JSON, Write-Output in text mode. Mirrors the bash wording and stream routing. One source file.

Testing

• uvx ruff check clean; tests/test_ps1_encoding.py green (edited .ps1 stays ASCII / PowerShell 5.1-safe).
• New test_ps_setup_plan_copied_message_on_stderr_in_json_mode mirrors the existing bash test_setup_plan_json_parseable_on_first_run: asserts -Json first-run stdout is parseable JSON and Copied plan template is on stderr.
• Verified fail-before / pass-after locally with Windows PowerShell 5.1: before, stderr is empty and the test fails; after, the message is on stderr and stdout remains pure JSON ({...,"BRANCH":""}). 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 located the missing status message across the bash/PowerShell twins and drafted the fix plus a mirroring regression test; I confirmed the bash wording/stream routing, verified fail-before/pass-after under Windows PowerShell 5.1 (stdout stays pure JSON), and reviewed the diff before submitting.