fix: what an audit of the documented behaviour found

An independent check of every behaviour AGENTS.md was to describe:
- `--units ,` and `--only ,` skipped their empty items and so had no
  filter left: a typo started a run of every unit. They are usage errors
  now (exit 3).
- Two fixes the plan lists were missing: a dry run now clears the prompts
  a dry run before it left in the dir they share (the report counted
  them), and `config` reports an absolute functional.mcp.command that is
  no program here - /usr/bin/npx inside the Flatpak, whose /usr is the
  Sdk's.
- A patch that is not UTF-8 stops `run` at preflight (exit 3), as the
  plan has it; it exited 1.
- An exception no command caught aborted the process (134), e.g. a home
  that holds no lenses: it ends with its message and exit 1. An error
  escaping `run` or `freeze` now also clears the signal handler's run and
  sweeps what is left below the subreaper.
- `config -h` printed "unrecognized arguments" (exit 3): it prints the
  usage, exit 0.
- An API seat's .claude.json is linked into place, so a second writer
  never replaces the first's file and what claude wrote to it since.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Config/main.cpp

@@ -64,6 +64,12 @@ int main() {
           && Any(problems, "aiTrailers") && Any(problems, "authors"), Show(problems));
     problems = bad({{"functional", {{"extraTools", {"rm -rf /"}}, {"mcp", {{"args", "x"}}}}}});
     Check("extraTools must be Bash(<prefix>:*) or mcp__...; mcp.args a list", problems.size() == 2, Show(problems));
+    // A path found on the host is not there inside the Flatpak.
+    const std::vector<std::string> missingCommand = bad({{"functional", {{"mcp", {{"command", "/nonexistent/bin/npx"}}}}}});
+    Check("functional.mcp.command: an absolute path that is no program here is a problem; a name, or a program that is there, is not",
+          missingCommand.size() == 1 && missingCommand[0].starts_with("functional.mcp.command '/nonexistent/bin/npx' is no program here")
+              && bad({{"functional", {{"mcp", {{"command", "/bin/sh"}}}}}}).empty() && bad({{"functional", {{"mcp", {{"command", "npx"}}}}}}).empty(),
+          Show(missingCommand));
     std::ofstream(p, std::ios::binary) << "{not json";
     Check("not JSON -> one problem", load().problems.size() == 1);
     Check("--config to a missing file is a problem", Any(Config::Load(td, td / "none.json").problems, "no such file"));