[Phase 14] Document binary and source installation

[cosentinode]

Scope

Document archive download, checksum verification, extraction, PATH setup, source builds, runtime prerequisites, supported targets, and deferred package managers.

Acceptance Criteria

• Users can install each artifact using documented commands.
• Rust 1.85+, git, and optional gh prerequisites are explicit.
• bitbygit --version is the final check.
• README links installation docs without overstating package availability.

Verification

• cargo build --locked --release -p bitbygit
• cargo run -p bitbygit -- --version
• git diff --check

Notes

Child of #16; depends on final archive and publication contracts.