fix(plan): keep quickstart output out of implementation scope

[WOLIKIMCHENG]

Description

Clarifies that /speckit.plan should generate quickstart.md as a validation scenarios / run guide artifact, not as a place for full implementation code.

The updated prompt now asks for runnable validation scenarios, prerequisites, commands, and expected outcomes, while explicitly excluding full implementation code, model/service/controller bodies, migrations, and complete test suites.

Fixes #1400

Testing

• .venv/bin/python -m pytest tests/integrations/test_integration_generic.py::TestGenericIntegration::test_plan_defines_quickstart_as_validation_guide -q -p no:cacheprovider
• .venv/bin/python -m pytest tests/integrations/test_integration_generic.py -q -p no:cacheprovider
• git diff --check -- templates/commands/plan.md tests/integrations/test_integration_generic.py

[Copilot]

Pull request overview

This PR clarifies the intent of /speckit.plan so that quickstart.md is generated as a validation/run guide artifact (scenarios, prerequisites, commands, expected outcomes) rather than containing full implementation code. This aligns the plan output with the intended phased workflow (design/validation guidance first, implementation later via tasks.md).

Changes:

• Update the speckit.plan command template to explicitly define quickstart.md as a runnable validation guide and explicitly exclude implementation code/migrations/full test suites.
• Add an integration test asserting the generated speckit.plan.md includes the new quickstart guidance wording.

Show a summary per file

File	Description
templates/commands/plan.md	Adds explicit quickstart validation guide instructions and excludes implementation scope in the plan prompt.
tests/integrations/test_integration_generic.py	Adds regression coverage to ensure the generated plan command keeps quickstart out of implementation scope.

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