docs: make user-facing guidance self-contained

[inureyes]

Summary

• Replace issue, pull request, and historical-version references in active CLI help, runtime warnings, manpages, README, current release notes, and public Rust documentation with durable behavior-focused guidance.
• Rewrite the exit-code strategy documentation and compression caveat in terms of current behavior and operator actions.
• Add help-rendering regression guards to bssh, bssh-server, and bssh-keygen so internal tracking references cannot silently return.
• Keep archived pre-3.0 changelog entries and private implementation comments unchanged for historical traceability.

Validation

• cargo test --workspace --all-targets with the configured localhost SSH key loaded into a one-shot agent
• cargo test -p bssh --lib help_uses_current_user_facing_terms
• cargo test -p bssh --bin bssh-server test_cli_parsing
• cargo test -p bssh --bin bssh-keygen test_cli_parsing
• cargo clippy -p bssh --lib --bins --all-features -- -D warnings
• cargo fmt --all -- --check
• Actual help audit across 16 root, subcommand, and pdsh-compatible paths

Risk and release

• This changes user-facing text and regression coverage only; command behavior, defaults, exit codes, and transport policy are unchanged.
• The published v3.0.0 release body was updated separately, while the existing tag and artifacts remain unchanged. Shipping these source and manpage updates requires a later patch release.