docs: add v0.5.0 release notes

docs/releases/v0.5.0.md

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+# sshkeeper v0.5.0 — Persistent SSH Sessions
+
+v0.5.0 adds an optional multi-session workflow backed by `tmux`. It lets
+sshkeeper keep several interactive SSH connections alive in one terminal
+workspace without turning sshkeeper itself into a terminal emulator.
+
+`tmux` is intentionally optional. If it is not available, Sessions are not
+shown anywhere in the TUI and ordinary Connect/Tunnel workflows behave exactly
+as they did in v0.4.1.
+
+## Persistent sessions
+
+When `tmux` is available in `PATH`:
+
+- **Server Actions → Open in session** opens the selected server in a persistent
+  tmux window;
+- **Manage → Sessions** lists SSH windows created by sshkeeper;
+- `Enter` attaches to the selected session;
+- `Ctrl+D` closes it after confirmation;
+- `Ctrl+R` refreshes the list.
+
+If sshkeeper runs outside tmux, it uses a dedicated tmux workspace named
+`sshkeeper`. If it is already running inside tmux, new SSH windows are created
+inside the current tmux session rather than starting a nested client.
+## Vault and authentication
+
+Sessions continue to use the existing sshkeeper/OpenSSH connection planner,
+including routes, bastions, identity files and startup commands.
+
+Key and SSH-agent sessions do not need a vault unlock. Password and
+key-passphrase sessions ask for the vault master password inside their own tmux
+window. Secrets are not copied through command-line arguments, environment
+variables or temporary shell scripts.
+
+## Platform behavior
+
+Linux and macOS support Sessions when `tmux` is installed. On macOS it can be
+installed with Homebrew using `brew install tmux`.
+
+Native Windows builds keep Sessions hidden because upstream tmux is not a native
+Windows backend for this workflow. Windows users can use Sessions by running the
+Linux build inside WSL with tmux installed there.
+
+Linux native packages do **not** require tmux. Package metadata keeps OpenSSH as
+the hard dependency and marks tmux only as `Recommends`, so the application
+remains fully usable without the Sessions feature.
+## Validation
+
+The release is covered by normal unit tests plus a real tmux lifecycle test that
+creates a temporary workspace/window, verifies sshkeeper can discover its
+metadata, closes it, and confirms it disappears.
+
+The release gate also runs `go vet`, Linux package migration tests and release
+cross-builds for Linux amd64/arm64, macOS amd64/arm64 and Windows amd64. GitHub
+CI additionally runs the test suite natively on both Ubuntu and macOS.
+
+## Install
+
+Debian/Ubuntu (amd64):
+
+```bash
+sudo apt install ./sshkeeper_0.5.0-1_amd64.deb
+```
+
+Fedora/RHEL-family (x86_64):
+
+```bash
+sudo dnf install ./sshkeeper-0.5.0-1.x86_64.rpm
+```
+
+ARM64 packages and tar/zip archives are published alongside them. Verify
+downloads against `checksums.txt`.