chore: prepare v0.26.1 release

Bump all-smi to 0.26.1 across Cargo.toml, Cargo.lock, the manpage, and Debian changelog, and add the README "Recent Updates" entry.

Cover the 9 commits since v0.26.0: keychain-independent macOS signing and packaging checks (#385, #387), complete recovery dispatch and promotion paths (#386, #388), early Homebrew credential validation and stale-version diagnostics (#389, #390), the Rust dependency refresh (#391), and protected native Windows checks (#392).

Validated with cargo fmt --all --check, cargo check --all-targets --all-features, cargo clippy --all-targets --all-features -- -D warnings, and cargo test --all-targets --all-features.