Simplify config handling around fixed aindex paths

[TrueNine]

Summary

• remove the CLI config command surface and stop exposing fixed aindex path overrides as user config
• keep the aindex layout fixed internally and centralize the Rust-side defaults for reuse in GUI/runtime code
• update GUI validation/forms and docs to describe the fixed <workspaceDir>/aindex structure clearly

Testing

• pnpm exec vitest run src/ConfigLoader.test.ts src/config.test.ts
• pnpm exec vitest run src/utils/configValidation.test.ts src/utils/configValidation.property.test.ts
• pnpm exec vitest run src/pipeline/CliArgumentParser.test.ts
• pnpm run check:type
• cargo test -p tnmsc --lib core::config::tests
• cargo check -p tnmsc-cli-shell
• cargo test --manifest-path gui/src-tauri/Cargo.toml commands::tests

Note

• pnpm exec tsc --noEmit in cli/ still hits the pre-existing type error in src/tsdown-config.test.ts.