fix(install): install a repository once, by the first hook that supports it

The pipeline used to skip a second installer only when it advertised
capabilities the first had already provided, so a hook with a different
capability still ran on the very same project. file-dedupe was installed
through pip and then again through `make install`, whose first line upgrades
pip; PEP 668 refuses that, and the run went red on a repository that was
already installed correctly.

One hook now installs a repository and the pipeline stops there. The order of
INSTALLERS is the preference: `make install` leads, because it is the hook a
repository writes for itself, while the others infer an installation from a
manifest.

A hook that fails installed nothing, so the rule is one *successful*
installation rather than one attempt: _run_installer returns a bool instead of
re-raising, and the caller moves on to the next hook. To keep that fallback
from turning a broken repository into a silent pass, a repository whose hooks
all fail now exits with the list of what was tried.

The capability bookkeeping the old skip rule needed is gone from the pipeline;
discover_capabilities remains on BaseInstaller and is now unused there.

Verified: 14 targeted unit and integration tests, and a full `pkgmgr update
--all` sweep in the arch image with 0 installation failures. Delta measured at
the consumer: repositories left without a CLI entry point, 16 before and 16
after, diff identical.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/integration/test_recursive_capabilities.py

@@ -1,9 +1,5 @@
 """
-Integration tests for recursive capability resolution and installer shadowing.
-
-These tests verify that, given different repository layouts (Makefile, pyproject,
-flake.nix, PKGBUILD), only the expected installers are executed based on the
-capabilities provided by higher layers.
+Integration tests for installer selection across repository layouts.
 
 Layer order (strongest → weakest):
 
@@ -129,10 +125,8 @@ class TestRecursiveCapabilitiesIntegration(unittest.TestCase):
             "With only a Makefile, the MakefileInstaller should run exactly once.",
         )
 
-    def test_python_and_makefile_both_run_when_caps_disjoint(self) -> None:
-        """
-        If Python and Makefile have disjoint capabilities, both installers run.
-        """
+    def test_only_the_first_supported_hook_runs(self) -> None:
+        """A repository with two usable hooks is installed by the first one."""
         repo_dir = self._new_repo()
 
         # pyproject.toml without any explicit "make install" hint
@@ -153,9 +147,8 @@ class TestRecursiveCapabilitiesIntegration(unittest.TestCase):
 
         self.assertEqual(
             called,
-            ["python", "makefile"],
-            "PythonInstaller and MakefileInstaller should both run when their "
-            "capabilities are disjoint.",
+            ["python"],
+            "Only the first supported hook may install the repository.",
         )
 
     def test_python_shadows_makefile_when_pyproject_mentions_make_install(self) -> None: