Stabilize V1 contracts, desktop support, and release hygiene

[Bayern99]

Problem

llm-pack claimed a credible local V1 surface, but the shipped state still had contract drift, a broken desktop shell compile, and weak GitHub-facing maintainer hygiene.

Scope

• align report.json contract sources to schema 0.3.0
• add schema-backed artifact validation tests
• restore XLSX formula warnings
• fix the desktop Tauri progress callback compile issue
• add a dedicated desktop CI gate
• rewrite public-facing maintainer and release docs

Out of scope:

• OCR, LibreOffice, format conversion, drag-and-drop, cloud workflows
• any stricter QUALITY_DEGRADED exit policy

Contract Changes

• report.json schema version is now consistently 0.3.0
• report.json.output.package_dir is documented as a filesystem path to the package root
• package-internal artifact paths remain relative
• warnings are verified to survive into both document.json and report.json

Validation

./scripts/validate-schemas.sh
./scripts/check-fixtures.sh
cargo fmt --all -- --check
cargo clippy --workspace -- -D warnings
cargo test --workspace
npm --prefix apps/desktop ci
npm --prefix apps/desktop run build
cargo check --manifest-path apps/desktop/src-tauri/Cargo.toml

All passed locally.

Notes

• Manual desktop GUI smoke was not run; this PR validates desktop through frontend build and src-tauri compile health only.
• GitHub About metadata was updated out of band to add a description and focused topics.
• CHANGELOG.md and RELEASING.md were added so future releases can copy directly from the tracked changelog.