build: Review Hex package contents and include docs extras

[zacky1972]

Summary

Addresses #49 by making Hex package contents explicit and verifiable before the v0.1.0 release.

Changes

• Adds an explicit package[:files] allowlist via package_files/0.
• Includes the docs/ directory so ExDoc extras such as docs/module-responsibilities.md are present in the Hex package.
• Excludes local and generated artifacts from the Hex package by omission, including priv/plts, _build, deps, doc, tmp, cover, .elixir_ls, test, and maintainer scripts.
• Adds docs/hex-package-contents.md to document included and excluded package paths.
• Adds mix package.contents, which builds and unpacks the Hex package and verifies required and forbidden paths.
• Adds tests for package[:files], docs[:extras], and the package contents alias.

Validation

Run locally:

mix format
mix test test/lean_lsp/package_contents_test.exs
mix package.contents
mix check

mix package.contents should unpack the package under _build/hex_package_contents and report that required release files are present while local/generated artifacts are absent.

Notes

This change intentionally does not modify README.md, including the final License section.