Add versioned templates to stack init

[konojunya]

Summary

• add a non-interactive stack init command with a safe hello-stack default, nine curated template IDs, explicit output selection, and opt-in atomic replacement through --force
• embed the catalog and exact source bytes from the pinned public Stack specification corpus, with CI drift checks and provider-pack guidance for branded templates
• verify every template through end-to-end initialization, stack check, and stack render, including existing-file protection and temporary-directory integration

Compatibility

• no language, engine, or file-format contract changes
• existing commands and exit status behavior remain unchanged
• initialization writes only the requested output path and never overwrites without --force

Validation

• cargo fmt --check
• cargo test --all-features --locked
• cargo +1.85.0 test --locked
• cargo clippy --all-targets --all-features --locked -- -D warnings
• cargo +1.85.0 clippy --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS=-D warnings cargo doc --no-deps --locked
• cargo llvm-cov --all-features --locked --fail-under-lines 90 --fail-under-functions 95 --fail-under-regions 90
• cargo build --release --locked
• python3 scripts/validate-render.py
• node scripts/validate-provider-catalogs.mjs
• pinned template snapshot and conformance validation against stack-sh/specification@8e5477c