Add inception bundled extension

[bigsmartben]

Summary

Adds a bundled inception extension for pre-iteration product and architecture inception workflows.

Changes

• Adds /speckit.inception.product and /speckit.inception.arch commands with command/template separation.
• Writes product and architecture inception artifacts under inception/, including UC, medium/high wireflows, capability analysis, API POC evidence, boundaries, domain model, and architecture pre-design.
• Makes inception part of the default bundled install and package contents.
• Updates default install smoke checks, extension registration tests, CLI integration tests, presets tests, README, and quickstart docs.
• Keeps API POC evidence tied to real code runs with required user confirmation before execution.
• Keeps mock strategy and standalone state model out of arch.md, with those details owned by later planning or domain-model.md as appropriate.

Validation

• .venv\\Scripts\\python.exe -m pytest tests\\extensions\\inception -q => 9 passed
• .venv\\Scripts\\python.exe -m pytest tests\\test_extensions.py tests\\test_extension_registration.py tests\\extensions\\inception -q => 354 passed, 3 skipped
• git diff --cached --check => passed

Disclosure

Prepared by Codex (model: GPT-5) on behalf of the repository maintainer workflow.