Prepare public release and SDK distribution

[techartdev]

The repository did not yet have a public documentation entry point, release automation, registry metadata, or the resource boundaries needed before broad public visibility. This change prepares the first public preview and keeps all completed local main work in the release branch.

• organizes and sanitizes the public documentation, and adds contribution, security, conduct, changelog, and release guidance
• adds pinned GitHub Actions for CI, Windows MSI/NSIS releases, npm trusted publishing, and crates.io publishing
• adds release metadata for @shellcanvas/app-sdk, shellcanvas-adapter-sdk, and shellcanvas-filesystem-sdk
• adds installation principals and bounded storage, clipboard, transfer-catalog, and adapter-package resource use from the pre-public security review
• builds unsigned Windows installers and publishes SHA-256 checksums; README and release docs disclose the current Windows warning

Validation:

• public repository gate: 603 files and 85 Markdown documents
• Vitest: 52 files, 291 tests
• cargo test --workspace --locked
• cargo clippy --workspace --all-targets --locked -- -D warnings
• npm run build, npm audit --audit-level=moderate, SDK verification, npm pack dry run, and both cargo publish dry runs
• local NSIS and MSI release builds completed successfully