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>

implementations/Crafter.Review-Config.cpp

@@ -10,6 +10,7 @@ import :Plan;
 import :Tools;
 import :PgWire;
 import :Process;
+import :Platform;
 import std;
 
 namespace fs = std::filesystem;
@@ -321,6 +322,18 @@ namespace Crafter::Review::Config {
         }
         const Json& args = Dyn::Get(Dyn::At(functional, "mcp"), "args");
         if (!args.is_null() && !AllStrings(args)) out.emplace_back("functional.mcp.args must be null or a list of strings");
+        // A path found on the host, such as /usr/bin/npx, is not there inside
+        // the Flatpak, whose /usr is the Sdk's.
+        const Json& command = Dyn::Get(Dyn::At(functional, "mcp"), "command");
+        if (command.is_string() && command.get<std::string>().starts_with('/')) {
+            std::error_code error;
+            const fs::path file(command.get<std::string>());
+            const bool executable = fs::is_regular_file(file, error) && (fs::status(file, error).permissions() & (fs::perms::owner_exec | fs::perms::group_exec | fs::perms::others_exec)) != fs::perms::none;
+            if (!executable) {
+                out.emplace_back(std::format("functional.mcp.command {} is no program here{}", Dyn::Repr(command.get<std::string>()),
+                                             Platform::InSandbox() ? " (inside the Flatpak /usr is the Sdk's: name it `npx`, the bundled one)" : ""));
+            }
+        }
         return out;
     }