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/docker/enumerate.py

@@ -74,8 +74,9 @@ def _query_services_by_project() -> dict[str, set[str]]:
             ],
             capture_output=True, text=True, check=False,
         )
-    except FileNotFoundError as exc:
-        raise EnumerationError("docker ps failed: docker not found") from exc
+    except OSError as exc:
+        # Missing, or on PATH but not executable by this user (PermissionError).
+        raise EnumerationError(f"docker ps failed: docker unavailable ({exc})") from exc
     if r.returncode != 0:
         raise EnumerationError(f"docker ps failed: {r.stderr.strip()}")
     return _parse_services_by_project(r.stdout or "")