Improve run_command error diagnostics with live output capture

Switch run_command to a single-run execution model that streams stdout/stderr
live while capturing both streams in memory using selectors. This guarantees
that command errors (e.g. make install, pip, nix) always show full diagnostics
without re-running commands or risking deadlocks.

Add unit tests for preview mode, success execution, failure handling, and
allow_failure behavior.

Context:
https://chatgpt.com/share/replace-with-this-conversation-link

src/pkgmgr/core/command/run.py

@@ -1,8 +1,10 @@
+from __future__ import annotations
+
+import selectors
 import subprocess
 import sys
 from typing import List, Optional, Union
 
-
 CommandType = Union[str, List[str]]
 
 
@@ -13,32 +15,97 @@ def run_command(
     allow_failure: bool = False,
 ) -> subprocess.CompletedProcess:
     """
-    Run a command and optionally exit on error.
+    Run a command with live output while capturing stdout/stderr.
 
-    - If `cmd` is a string, it is executed with `shell=True`.
-    - If `cmd` is a list of strings, it is executed without a shell.
+    - Output is streamed live to the terminal.
+    - Output is captured in memory.
+    - On failure, captured stdout/stderr are printed again so errors are never lost.
+    - Command is executed exactly once.
     """
-    if isinstance(cmd, str):
-        display = cmd
-    else:
-        display = " ".join(cmd)
-
+    display = cmd if isinstance(cmd, str) else " ".join(cmd)
     where = cwd or "."
 
     if preview:
         print(f"[Preview] In '{where}': {display}")
-        # Fake a successful result; most callers ignore the return value anyway
         return subprocess.CompletedProcess(cmd, 0)  # type: ignore[arg-type]
 
     print(f"Running in '{where}': {display}")
 
-    if isinstance(cmd, str):
-        result = subprocess.run(cmd, cwd=cwd, shell=True)
-    else:
-        result = subprocess.run(cmd, cwd=cwd)
+    process = subprocess.Popen(
+        cmd,
+        cwd=cwd,
+        shell=isinstance(cmd, str),
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        text=True,
+        bufsize=1,
+    )
 
-    if result.returncode != 0 and not allow_failure:
-        print(f"Command failed with exit code {result.returncode}. Exiting.")
-        sys.exit(result.returncode)
+    assert process.stdout is not None
+    assert process.stderr is not None
 
-    return result
+    sel = selectors.DefaultSelector()
+    sel.register(process.stdout, selectors.EVENT_READ, data="stdout")
+    sel.register(process.stderr, selectors.EVENT_READ, data="stderr")
+
+    stdout_lines: List[str] = []
+    stderr_lines: List[str] = []
+
+    try:
+        while sel.get_map():
+            for key, _ in sel.select():
+                stream = key.fileobj
+                which = key.data
+
+                line = stream.readline()
+                if line == "":
+                    # EOF: stop watching this stream
+                    try:
+                        sel.unregister(stream)
+                    except Exception:
+                        pass
+                    continue
+
+                if which == "stdout":
+                    stdout_lines.append(line)
+                    print(line, end="")
+                else:
+                    stderr_lines.append(line)
+                    print(line, end="", file=sys.stderr)
+    finally:
+        # Ensure we don't leak FDs
+        try:
+            sel.close()
+        finally:
+            try:
+                process.stdout.close()
+            except Exception:
+                pass
+            try:
+                process.stderr.close()
+            except Exception:
+                pass
+
+    returncode = process.wait()
+
+    if returncode != 0 and not allow_failure:
+        print("\n[pkgmgr] Command failed, captured diagnostics:", file=sys.stderr)
+        print(f"[pkgmgr] Failed command: {display}", file=sys.stderr)
+
+        if stdout_lines:
+            print("----- stdout -----")
+            print("".join(stdout_lines), end="")
+
+        if stderr_lines:
+            print("----- stderr -----", file=sys.stderr)
+            print("".join(stderr_lines), end="", file=sys.stderr)
+
+        print(f"Command failed with exit code {returncode}. Exiting.")
+        sys.exit(returncode)
+
+    return subprocess.CompletedProcess(
+        cmd,
+        returncode,
+        stdout="".join(stdout_lines),
+        stderr="".join(stderr_lines),
+    )