feat: improve sshrm argument handling and help output

.continue/rules/project.md

@@ -0,0 +1,20 @@
+---
+description: SSHRM project conventions
+---
+
+# Project conventions
+- Use English throughout the project.
+- Keep shell scripts Bash-based when Bash is already used by the project.
+- Preserve the current behavior of the main scripts:
+  - `sshrm`: remove an entry from `~/.ssh/known_hosts` by host name or by line number.
+  - `git-release`: handle releases from `dev` to `main`, create an annotated tag, and push commits and tags.
+- Keep user-facing messages short, clear, and in English.
+- Prefer minimal, focused changes that do not alter the intent of the existing scripts.
+- Maintain `.continue/rules/project.md` whenever project conventions or script behavior change.
+- `sshrm` is implemented as a small Bash script with helper functions, while preserving host and line-number removal behavior.
+- `sshrm` should print a short usage line, support `-h`/`--help`, and fail clearly on missing or invalid line-number input.
+
+# Project identity
+- Main script: `sshrm`
+- Release helper: `git-release`
+- License: GNU GPL v3