Add CLI configuration diagnostics

[konojunya]

Summary

• add read-only stack config path and stack config get default_icons_path discovery commands
• add stack doctor checks for version, config source and validity, icon-store source, and known-provider packs with redacted actionable diagnostics
• integrate help, completions, manual assets, release verification, documentation, and source-versus-published Aqua CI boundaries

Verification

• cargo +stable test --locked
• cargo +stable clippy --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +stable doc --no-deps --locked
• cargo +1.85.0 test --locked
• cargo +1.85.0 clippy --all-targets --all-features --locked -- -D warnings
• cargo +stable llvm-cov --all-features --locked --fail-under-lines 90 --fail-under-functions 95 --fail-under-regions 90
• python3 scripts/generate_cli_assets.py --binary target/debug/stack --check
• distribution, supply-chain, workflow, and release-script test suites
• checksum-locked Stack CLI 0.4.0 Aqua archive generator parity