fix(doctor): survive a PATH entry the user can't execute

The install itself now works end to end on a fresh macOS account — venv built,
package installed from git, entry point linked — and `doctor` then died with an
unhandled traceback:

    PermissionError: [Errno 13] Permission denied: 'ip'

netpool's probe helpers caught only FileNotFoundError. That is not the only way
a probe binary can be unavailable: when a name on PATH exists but this user
cannot execute it, exec fails with EACCES, and CPython reports that in
preference to the ENOENT from the other PATH entries. So `except
FileNotFoundError` misses it and the crash propagates all the way out of
`doctor`. Reproduced directly: a mode-000 file named `ip` on PATH yields
exactly the error above.

_run_ok's docstring already stated the intent — treat an unavailable binary as
failure rather than crashing — so this widens the catch to OSError to match
what it says. Any OSError means the probe could not run, which for a
fail-closed check is indistinguishable from "not present". The same narrow
catch is fixed in overlapping_routes and in the two docker probes
(compose ls, docker ps), which are the same shape and equally reachable.

Deliberately not touched: the FileNotFoundError catches around file I/O in
bottle_state and orchestrator/service, where the narrow exception is correct.

The harness also required `bot-bottle` on PATH before running doctor, which
could never be true: install.sh prints the PATH line rather than editing a
shell profile, by design, so on a fresh account the entry point is installed
and working but not on PATH. It now looks where the installer actually puts
it (~/.local/bin, then the venv) and notes when it's running by absolute path.
That was the harness failing a run for a reason the installer intends.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WEfZZhakx13bxTfXcZCoS5

bot_bottle/backend/firecracker/netpool.py

@@ -177,14 +177,20 @@ def gw_slot() -> Slot:
 # --- fail-closed verification ---------------------------------------
 
 def _run_ok(argv: list[str]) -> bool:
-    """Run a probe command, treating a missing binary as failure
+    """Run a probe command, treating an unavailable binary as failure
     (rather than crashing) so callers can stay fail-closed."""
     try:
         return subprocess.run(
             argv, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
             check=False,
         ).returncode == 0
-    except FileNotFoundError:
+    except OSError:
+        # Not only "missing". A name on PATH that isn't executable by this
+        # user raises PermissionError, and CPython reports that EACCES in
+        # preference to the ENOENT from the other PATH entries — which is
+        # how `doctor` came to die with a traceback on a fresh macOS
+        # account. Any OSError means the probe couldn't run, which for a
+        # fail-closed check is indistinguishable from "not present".
         return False
 
 
@@ -244,7 +250,9 @@ def overlapping_routes() -> list[RouteConflict]:
             ["ip", "-json", "route", "show", "table", "all"],
             capture_output=True, text=True, check=False,
         )
-    except FileNotFoundError:
+    except OSError:
+        # Missing, or present-but-not-executable for this user; either way
+        # there are no routes we can enumerate. See _run_ok.
         return []
     if proc.returncode != 0 or not proc.stdout.strip():
         return []