refactor: split into idx-core library and idx-cli binary workspace

Phase 2 of docs/DESIGN.md, no behavior change:

- crates/idx-core (library): api, analysis, cache, config file handling
  and provider selection, curl_impersonate, error, ownership, runtime,
  plus the shared test fixtures. clap derives are behind an optional
  `clap` feature so the library does not require clap.
- crates/idx-cli (binary `idx`): cli/, output/, and IdxConfig (output
  format and color resolution). Package and binary names are unchanged.
- config.rs is split: file read/write and validation move to core; a
  CLI test keeps core's OUTPUT_VALUES in sync with OutputSetting.
- ownership::db::open_db/db_path drop their unused IdxConfig argument.
- One workspace version; shared dependency versions; no third-party
  version changes in Cargo.lock. LICENSE is symlinked into each crate.
- flake: workspace version, new source paths, and a per-crate package
  check (idx-cli is built from its tarball against packaged idx-core,
  since idx-core is not on crates.io yet). CI cache key, release path
  filter, and the golden whitespace-hook exclusion follow the new layout.

Parity: all 143 golden outputs match byte for byte without
regeneration; 157 core + 17 CLI unit tests (173 before plus the new
OUTPUT_VALUES sync test), 62 CLI tests pass. Live A/B against main:
16/16 stocks cases identical; ownership discover got IDX 403 on both
(exit 4 vs 1 is the phase 1 exit-code change).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
       - name: Restore and save Nix store
         uses: nix-community/cache-nix-action@v7
         with:
-          primary-key: nix-v2-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('flake.nix', 'flake.lock', 'Cargo.lock', 'Cargo.toml') }}-${{ hashFiles('src/**', 'tests/**', 'scripts/**', 'install.sh', 'README.md', 'LICENSE') }}
+          primary-key: nix-v2-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('flake.nix', 'flake.lock', 'Cargo.lock', 'Cargo.toml') }}-${{ hashFiles('crates/**', 'scripts/**', 'install.sh', 'README.md', 'LICENSE') }}
           restore-prefixes-first-match: |
             nix-v2-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('flake.nix', 'flake.lock', 'Cargo.lock', 'Cargo.toml') }}-
             nix-v2-${{ runner.os }}-${{ runner.arch }}-